Sim's Electrical Companion
Protective Devices - Service Head
(HBC / HRC) Hight Breaking / Rupture Capacity Fuses
The supply cables to a property are protected by the cut out in the service head / Henley Block. This holds a fuse for each supply phase. The purpose of this fuse is not to protect the building so much as to protect the supply cable from faults that may occur in the building. It was a practice up until 1955 to have a fuse also on the /neutral conductor. This is not safe on an AC supply so if this is the case inform the DNO as it is no longer practiced. If the neutral is open circuit you may be led to believe the installation is isolated when it is not.
As the name suggests the main fuses are capable of breaking large fault currents without external physical damage. They are constructed in a way that breaks down large fault currents into smaller arcs which are extinguished.
Operation & Constructional Features of HRC/HBC Fuses
The fuse is connected in the line conductor by metal end caps (1). In the case of overload or short circuit the unhealthy circuit or equipment will draw more current than the fuse elements (2) are designed for. In the centre of the elements is a weak spot (3) made of silver that will melt at a designated set point of over current. There are a few fuse elements (2) which have the effect of reducing the current in each element. When the element breaks this will cause multiple arcs to occur rather than one very large arc if there was only one element. The ceramic fuse body (4) contains the explosive effect of the overload and the arc is extinguished by silica (sand) balls (5). In some cases the fuse may have an indicator bead to show if it has operated (6).
Commonly in older properties the fuse is a BS 1361. In 2015 this standard was withdrawn and replaced by the more robust BS 88 type fuses. The older BS 1361 have far lower breaking capacities and due to higher consumer demand and fault currents due to improved earthing arrangements the BS 88 has now taken its place. The likelihood is BS 1361 fuses will still be in operation for a couple of decades but upgrades to any system supplied by them are likely to be replaced with the new type BS 88's.
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Generally BS 1361's were used for single phase domestic purposes and BS 88's were used for commercial or industrial three phase supplies less than 1000V AC. The BS 88 is a more durable fuse and has now been chosen for domestic purposes as well due to improvements in supply systems which increase the potential fault currents. Interestingly the fault curve for BS 1361 fuses has been removed from BS 7671 electrical regulations. To make comparisons earlier versions of the regulations need to be referenced or manufacturers data.
Types of BS88 Fuse
A-J The type refers to the physical form e.g. bladed, bolt on single, twin bolts, and its size etc. These fuses are designed to prevent accidental replacement with over / under rated replacements.
gG General use where current is relatively regular and loading is mostly resistive.
gM Motors or other devices with high start up current or peaks in current.
Main Switch
After the meter the first point of control on the consumers side of the installation is normally a main switch. It's purpose is to give the consumer a method of safely isolating all final circuits from the source of supply. The main switch is NOT a protective device and should never be operated while the final circuits are loaded. The reason I have included this here is to make an important point. The main switch is a form of protection but it is not automatic it is purely to manually isolate circuits so they can be worked on safely.
In protective devices there is a method of reducing the arc caused by high load switching. The main switch does not have this feature so it can be quite drastic turning it off / on under load. Most modern main switches are to the British Standard BS 60947-3 and should survive a fault current of up to 16kA but it is still not advisable to switch under load as it can degrade the contacts by blackening and pitting the contacts which will reduce the life span of the isolator, increase resistance and lower disconnection times as a result.
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The main switch is purely for isolation and compared to a protective module of the same size the weight gives a good idea that there is far less inside. The point of the linked isolator is to isolate both the line and neutral in single phase applications as shown below. On three phase systems it may be a linked switch that isolates the three phases in a 3 module isolator or three phase and neutral (TP-N) on a 4 module switch.
BS 3036 - Semi Enclosed Fuse
Introduced in 1958 the only circuit protection that was in common use was the use of fuse wire or cartridge fuses. Their method of operation is simple the more current being drawn the greater the temperature of the fuse element. Using this method the element is designed to disconnect at a certain prescribed limit dependant upon its material & cross sectional area and the current drawn across it and the time period.
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Many domestic installation in the UK are still protected by rewireable fuses. These are not very effective at disconnecting overload faults which is why there is a current downrating of cables protected by these fuses of x 0.725 (Cf ) this is covered in more detail in the section on cable selection. It is not a requirement to keep an electrical installation up to the very latest regulations for good reason, it is not practicable, however it is encouraged. There are a few reasons why it should be encouraged to upgrade to a more modern method of protection.
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BS 3036 fuses can be mistakenly or purposefully rewired with the incorrect size fuse wire or other conducting material. It is not a simple task to rewire a fuse in the dark! If you do have rewireable fuses in your home I recommend you become familiar with what circuit each fuse is protecting. Clearly label them, as they rarely ever are. Leave a pack of fuse wire, a flat head screwdriver and a torch within reach of the fuse-board. You will thank me for that advice when you blow a fuse.
Correctly installed fuses are color coded to indicate what current they are designed for. The fuse holder has identification spots on it and the fuse carrier will be of the same colour. The pins vary in size to match the carrier.
The other drawback of our 60 year old friend is they have a very low breaking capacity compared to modern devices like circuit breakers. The arc created by the fuse wire as it breaks can literally blow the fuse carrier apart if the fault current is too high. With modern earthing systems or upgraded supplies it may be the case that the fuses need to be replaced with a more robust method of protection.
* S = Semi enclosed
** Rating should be recorded as 1kA if true value cannot be confirmed. See manufacturers information for more details.
BS3871 Cicuit Breakers
Thermal & Magnetic Operation of Circuit Breakers
Compared with a rewirable fuse the circuit breaker is streets ahead in its sensitivity, operation and speed of resetting. Instead of getting out your trusty screwdriver all you need to do is push a button or lever to reset a circuit once the fault has been rectified.
There are two methods of disconnection employed in these breakers and their breaking capacity where they remain serviceable is higher than that of a semi enclosed fuse. The two methods of disconnection are thermal and magnetic. The thermal operation is operated by a bi-metallic strip. When current is drawn through the strip it heats up and as the metals are dissimilar one will expand more than the other. They are bonded to each other which has the effect of bending the metals which is capitalised upon by making or breaking a contact. This method is used to open a circuit in overload. An overloaded circuit is not necessarily unhealthy it may be that too many items are plugged into it. Overload takes time to operate as it does rely on the heating of the bi-metallic strip.
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To cope with much larger fault currents in the case of a serious fault like short circuit there is another function provided by the circuit breaker - Magnetic trip. There is a coil in the circuit breaker that acts like a solenoid. If the magnetic field it generates is high enough it will open the circuit and disconnect it. The circuit is then mechanically held open until it is physically reset. The coil can be made to any size required so the value of fault current can be carefully calculated to give an accurate device capable of disconnecting a short circuit in a very short period of time.
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The addition of arc splitters inside the device also improves its ability to withstand fault currents by breaking them into more manageable arcs. The short circuit capacity (Icn) that a circuit breaker can effectively deal with is denoted by the symbol M and the rating after it. To see fault curves showing time/current characteristics of BS 3871 circuit breakers you will need to use internet archives, manufacturers information or use BS 7671 15th edition. The tables below show some common values for time characteristics and short circuit capacity.
Components of a BS 3871 Circuit Breaker
1 Manufacturers data and specifications
2 Circuit close push button (ON)
3 Circuit open / test button (OFF)
4 Supply Line blade connector
5 Bi metallic strip for overload protection
6 Magnetic trip coil for short circuit protection
7 Arc splitter & arc chamber to reduce arc amps
8 Open / Close contacts & mechanism
9 Load line blade connection
BS60898 Circuit Breakers
Overload & Short Circuit Protection
BS 60898 is split into two subgroups type 1 and type 2. BS 60898-1 is for AC circuits only and BS 60898-2 is for AC & DC circuits. In the trade these are often referred to as MCB's (Miniature Circuit Breakers) this is a nick name at best and is not a true definition. The BS 60898 standard replaces the earlier standard BS 3871. Although there are some similarities in operation the time characteristics are not the same as the new BS 60898. For example a B type breaker will operate in fault conditions in time if it has more than 3-5x its Operating current flowing through it. Lower fault current above its rated current will take time to disconnect due to the time taken to heat up the bi-metallic strip.
When the newer standards were in the process of being changed over manufacturers were allowed to use the new BS 60898 designations on the old BS 3871 devices if they were within the relative new standard times.
Type 1 2.7 - 4x Could be defined under type B < 5x
Type 2 & 3 < 8 - 10x Could be defined under type C < 10x
Type 4 10 - 50x No BS 60898 equivalent standard as a type D < 20x
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These circuit breakers work in essentially the same way as the BS 3871's. In overload conditions a bimetallic strip will heat up and at a determined point will disconnect the circuit until the reset lever is manually operated and the strip has cooled down. In higher fault conditions like short circuit a magnetic solenoid mechanism will automatically disconnect the circuit which also can only be reset manually once the fault has been removed.
As shown in the figure short circuit capacity Icn is shown in a rectangle & below that is the energy limiting class value (I²t). Icn is the maximum fault current that can be disconnected without destroying the device. The devices are tested to this upper limit and it should never be exceeded.
Icn values for BS60898 circuit breakers are commonly: 1.5kA, 3kA, 4.5kA, 6kA, 10kA, 15kA, 20kA & 25kA.
Typical ratings in amperes of these circuit breakers are: 3, 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 & 125A.
Energy limiting class (I2t) is to assist the designer in achieving selectivity between protective devices. This tells the designer how much current will be let through the device during fault conditions allowing them to consider the choice of protective devices up / down stream.
For BS 60898 the energy limiting class is stated as 1, 2 or 3 the highest is 3.
To incorporate the energy limiting class in design calculations reference needs to be made to manufacturers instructions / data sheets as they vary between different manufacturers.
Components of a BS 60898 Circuit Breaker
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1 On / Indicator (Red = On & Green = OFF)
2 Manual control lever
3 Supply Line terminal (bus bar connection)
4 Bi metallic strip for overload protection
5 Magnetic trip coil for short circuit protection
6 Arc splitter to reduce arc amps
7 Arc chamber
8 Open / Close contacts & mechanism
9 Circuit live terminal (final circuit)
10 Adjustment grub screw
11 Load terminal
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Manufacturers specification is on the side or face of the device.
Fault Current Curves
In appendix 3 of BS 7671 and published in manufacturers data sheets is the fault current and time characteristics of individual circuit breakers and fuses. The graph is on a logarithmic scale so it fits more comfortable in the room you are reading it in! Each row and column section goes up by a multiple of 10 for each block allowing very small and large values to be counted in the same graph without too much unnecessary space.
Example data that can be taken from the fault curve:
At 1.5x rating disconnection is as much as 150 seconds
4x rating disconnection is approx 20 seconds
Magnetic trip will occur at 5x rated current
BS60947-2 Circuit Breakers
MCCB - Moulded Case Circuit Breakers
Used for industrial and commercial circuit protection these circuit breakers should be under the supervision of electrically competent persons. These protective devices offer magnetic and thermal protection in a similar way to BS 60898's but they have a larger current range and often have adjustable protection settings which need to be understood by the operator.
Current ranges are from 0.5A - 6400A with short circuit capacities up to 100 kA.*
The inset picture shows four of these circuit breakers in the off / open position. Notice there are three positions unlike a domestic type circuit breaker. The extra position is to indicate that the device has operated automatically due to a fault being detected rather than just being turned off by an operator.
The time current characteristics graph on the left shows that this circuit breaker has a time characteristic similar to a B type BS 60898 circuit breaker. The notable difference is it is not a single line. The block gives some indication of the range of adjustment that is possible.
A = The minimum characteristics
B = The maximum adjustable thermal operation
The circuit being protected by this type of circuit breaker may have a characteristic that requires the operator to decrease the devices sensitivity to avoid nuisance operation / tripping in normal operation.
Example equipment that may require adjustment can include but is not limited to motors that do have an initial inrush current. They also draw more current until the motor has run up to speed.
You can liken this effect to pushing a car. It may take three or four people to get the car rolling but once it is rolling less energy is required to keep it rolling.
In this case during the starting process the protective device could detect the initial effort and disconnect as if it were a fault current.
As well as providing magnetic and thermal fault protection they can include additional features including undervoltage, residual current, time delays, adjustable levels of protection and remote open/close and indication of operation. Additionally in some cases they can report time of operation and cases of automatic / manual release which can assist the operator to maintain the system and have details of its service life.
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* Information from several manufacturers so be aware their published data varies and should be checked.
Air, Vacuum, Oil & SF6 Circuit Breakers
Used predominantly in Supply and distribution systems & industrial applications. They are produced to the same guidelines given by BS 60947-2 but there are some notable differences. The main difference is in their construction and method or arc extinguishing. The short circuit withstand value I²t is designed to provide selectivity with downstream devices.
As with smaller breaker types there is a method of arc splitting to reduce the single fault arc that would occur between contacts if it was not there. This is the main difference between air, vacuum and SF6 breakers.
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Air Circuit Breakers (ACB's)
With an air circuit breaker they capitalise on electromagnetic and thermal effects during an arc to assist in the process of extinguishing it. The arc is channeled into an arc chamber above the contacts and uses air as an insulator. The arc chamber splits the arc and helps dissipate the heat produced.
They are for low voltage applications below 450V and are commonly rated between 800A - 10kA. ACB's can be manually or electronically operated from and alternative source of power. Manual operation requires spring charging an internal motor before turning on.
Air Blast Circuit Breakers
These work in a similar way to Air circuit breakers but to achieve faster extinguishing of the arc compressed air is blasted at the arc. To achieve extinguishing of arcs on supplies of up to 765kV multiple air blasts are used to extinguish the arc.
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Oil Circuit Breakers
Used for breaking voltages up to 275kV. They are used for transmission and distribution systems. In many cases these are the oldest type of industrial circuit breaker and in general are being superseded by SF6 or vacuum types though some modern types have been produced. They rely on the oil creating a bubble around the arc that quenches it. These are easily recognised by the large cylinder that contains the oil with connections made into the top of the cylinder.
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Vacuum Circuit Breakers
Due to the high dielectric strength* of a vacuum it is used to extinguish the arc. The lack of oxygen helps to suppress the arc. These are used for medium voltage applications <66kV. A 1cm vacuum gap can withstand 200-400kV.
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* Dielectric strength is the ability of an insulator to resist carrying current where a potential difference exists.
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SF6 Circuit Breakers
Sulphur hexafluoride is the arc quenching medium. These circuit breakers are used for high voltage applications. SF6 has a very high dielectric strength. At the point of disconnection the arc chamber is flooded with high pressure SF6 to interrupt and extinguish the arc. SF6 is recognised as a greenhouse gas so use of it is controlled and waste / used gases must be collected and recycled or disposed of. Additionally it is expensive to make so it is only used for high voltage applications where it is the most suitable method.
Residual Current Devices - RCD's
In 1983 a new type of circuit protection was introduced. RCD's also known as RCCB's (Residual Current Circuit Breakers), operate when there is an imbalance between Live and Neutral conductors. This can only occur if there is an alternative path taken by the current to earth. They are easy to recognise by the test button on the front.
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The first type of RCD adopted by the wiring regulations was BS 5289 this standards operational speed at I delta n is lower than the new standard at ≤200ms. This had the effect of some nuisance tripping which in part led to the new standards BS 61008/9. These were designed to match module sizes of consumer units. Single phase units take up two modules for example. They increased the response time to ≤300ms to reduce disruption. These are not to be solely used for final circuit protection, there must be an additional form of overload protection.
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BS 61009 RCBO - Residual Current circuit Breaker with Overload protection. This is a single device that offers overload and earth fault protection in one unit. Single phase versions take up one or two modules in a modern board. Three phase RCBO's are packaged as 3 pole, 3 module units or 4 pole, 4 module TP & N units.
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There are many types of RCD and it can cause more problems than solving them if the wrong type is specified.
The table below identifies some specific type of RCD's for particular installations. Selection of the wrong type can cause nuisance tripping, loss of services or insufficient protection.
RCD Applications as Specified by BS 7671:2018
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In some cases RCD protection is a requirement of BS7671, where faults are more likely, RCD's are specified as an extra layer of protection. Most of these are specified for areas considered to be special locations.
Examples of special locations where RCD's are specified by BS7671:
Locations Containing a Spa, Bath, Pool or Fountain
Due to the increased chance of electric shock RCD protection is specified. The minimum level of protection is 30mA. This includes bathrooms but not WC's that are in a separate room from the bath / shower.
Rooms & Cabins Containing a Sauna Heater
All final circuits serving a sauna must be on 30mA RCD. The only item not required to be covered is the heater itself unless the manufacturers specify it.
Construction & Demolition Site Installations
Final circuits supplying sockets ≤32A must be protected by 30mA RCD. Socket outlets exceeding 32A should be protected by 500mA RCD.
Agricultural & Horticultural Premises
ALL final circuits need to be protected by 300mA RCD. Additionally socket outlets ≤32A must be protected by 30mA RCD. Socket outlets exceeding 32A should be protected by 100mA RCD. This allows sufficient discrimination / selectivity between circuits faults to minimise disruption to healthy circuits.
Photovoltaic Installations
If the PV converter is not able to protect the installation from DC disturbances an RCD type B 30mA is required. Refer to manufacturers instructions to confirm if this is a requirement.
Other areas where a 30mA RCD's are specified in PART 7 of BS 7671
In many cases the increased chance of shock risk requires additional protection including but not limited to RCD protection. See part 7 of BS 7671 for a full list of requirements. Locations that are included are listed below:
Circus' fairs & temporary stalls,
Floor and ceiling heating,
Fixed equipment in areas with restricted movement,
Cables concealed in walls <50mm without other method of protection,
Socket outlets feeding caravans, leisure tents, boats or on marinas,
Mobile electrical equipment,
Medical locations,
Exhibitions, shows & stands,
Outdoor lighting including street furniture,
Electric vehicle charging stations.
TT Earthing Systems
All TT systems have a higher risk of electric shock due to the variable resistance of the earth fault loop path. Due to this all TT systems must be protected by a minimum of 300mA RCD and overload protection for individual circuits. Final circuits connected to a TT supply that also need additional protection by RCD should be installed to prevent nuisance tripping and interruption of services so this is achieved by selecting more sensitive RCD protection downstream.
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RCD - Component Layout & Method of Operation
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The RCD is connected to the supply Line & Neutral (1). Current in the line and neutral will be equal & opposite so when you have a healthy circuit their magnetic fields cancel each other out. In a circuit that has leakage to earth (9) the magnetic fields in the line & neutral conductors will be out of balance and the difference creates a magnetic field in the toroid (5). This induces an emf in the trip loop (6) and the solenoid (7) opens both the live and neutral supply terminals. The reset lever (4) can be used to reset the RCD and the test button (3) should be operated every 6 months to keep the mechanism from seizing and ensure that it is working correctly.
The load terminals for live and neutral are at the bottom of the unit. Details on the type and specification are printed on the front or sides of the unit. Further details like I²t, Icn, maximum Zs and time characteristics can be found in manufacturers data sheets and BS 7671.
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RCBO - BS61009
Residual Current device & Overload
An RCBO provides the benefits of a circuit breaker & RCD in one package. Currently made to the standard
BS 61009 they combine the fault disconnection characteristics of circuit breakers to BS 60898 & RCD's to
BS 61008. The maximum earth fault loop impedance (Zs) is given by the RCD function.
If you are specifying all RCBO protection it is worth investing in a spacious consumer unit as most RCBO's are quite large compared to circuit breakers. Newer RCBO's have been developed that take up the same amount of space as a modern BS 60898 circuit breakers. These are becoming more common in the UK and are popular on the continent.
RCBO overload / short circuit characteristics are the same as their B, C, D etc type equivalent in Circuit breakers. The residual current function will be the equivalent of their RCD type e.g. AC, A, B, F etc.
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With this in mind there is a vast variety of RCBO for all manner of circumstances so specifying them requires some skill and good knowledge of the loading characteristics to avoid the pitfalls of under protecting specific loads or nuisance tripping.
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If the RCBO has a blue cable it must be connected to the correct neutral bar if you are using a high integrity / split load board. The cream cable connects to the earthing bar and is so coloured to show it is a functional earth not a protective earth.
Some manufacturers RCBO's do not have both of these conductors.
Constructional Features of an RCBO
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1 Manufacturers data and specifications
2 Trip / reset lever & mechanism
3 On (Red) off (Green) indicator
4 Supply Line terminal
5 Supply Neutral terminal
6 Functional earth connection (Cream)
7 Din rail release / lock slider
8 Line Load terminal
9 Neutral load terminal
10 Icn and current limiting class
11 CE or BS conformity logo
12 Voltage rating
13 Test button
14 Type & ratings for over current
& residual current
SPD - Surge Protection Device
With the growing reliance on electronic equipment that is susceptible to spikes in voltage the 18th edition regulations has introduced requirements to incorporate surge protection where needed. The provision of surge protection is determined by the likelihood of voltage spikes and the vulnerability of installed equipment. Surges are caused by on load switching and atmospheric effects namely lighting. The 18th edition regulations require installers of new circuits to conduct a risk assessment and determine whether surge protection is required to protect persons, property or livestock from the ill effects of a surge.
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Determining whether surge protection is required
To determine whether surge protection is a requirement an assessment needs to be made on the likelihood that a overvoltage, of atmospheric origin, will cause damage or danger in an installation. If the client requests that they do not want to have surge protection they must sign a disclaimer which takes the responsibility away from the installer.
How does it work?
A surge protection device is a single module that contains a Metal Oxide Varistor (MOV). This component acts similar to a Zener diode in the fact that it has very high resistance up to a certain voltage. However it works in both directions of current . The illustration above shows an SPD module and the voltage / current characteristics.
Uc Continuous operating voltage - The SPD will remain inactive
below this value.
In Nominal discharge current - The SPD can withstand this
current for 8-20 µs for at least 15 times.
Imax Maximum surge rating - The SPD may not be functional
having arrested this size surge. This will shown on the indicator panel
Up Voltage protection level at In (Nominal Discharge Current).
The illustration below shows how an SPD at the origin of an electrical installation may be installed. SPD's are installed parallel to the incoming supply. This one is being protected by a B32 circuit breaker as specified by the manufacturer. There are differences between different SPD manufacturers so follow their installation advice. When the SPD's operating voltage (Uc) is exceeded the excess is shed to earth to protect the electrical installation.
Surge Categories
Disruptive - may cause brief data corruption, system lock ups but usually comes back to full working function.
Dissipative - Repeated pulses which can severely hamper micro electronics. This can cause permanent damage communications faults and is disruptive at least.
Destructive - The surge is damaging to electronic equipment to the point of end of working life.
AFDD - Arc Fault Detection Device
The protective devices we have looked at so far will not detect the sudden changes that happen to a circuit during arc faults. Only very large parallel arc faults may be disconnected by overcurrent protective devices, RCD's or RCBO's. These are caused by poor connections, terminals, worn brushes, worn contacts and insulation damage.
Arc faults are split into two categories:
Parallel arcs - where the arc is between live conductors or between live conductors and earth
Series arcs are on a single current carrying cable i.e. break or loose connection in the line or neutral
During arc faults large amounts of heat can be generated which pose a fire risk or can cause equipment failure. An extra layer of protection can be added to help mitigate this risk. With the release of the 18th edition of the wiring regulations arc fault detection will be required where arc fault can pose a greater risk. In Canada and America AFDD's have been specified since 2002 for bedrooms and later most other types of power socket outlet similar to our uptake of RCD's.
It should be noted that there is a very mixed response from installers and companies that have had these installed. The primary cause for complaint is unexplained interruption of supplies through intermittent failures or possibly normal operation of motors or response to aberrations on shared supplies. Manufacturers of AFDD's all maintain that the fault detection is very smart at distinguishing between normal condition arcs and dangerous arcs. Normal condition arcing can be caused by brushed motors, contactors and on load switching.
The second amendment of the 8th edition specifies that AFDD's be installed for all final circuits up to 32A. The requirement is for Higher Risk Residential Buildings (HRRB) where the risk is higher due to difficulties in evacuation.
The requirements applies to:
Blocks of flats over 6 floors, (HMO's) Houses of multiple occupancy, purpose built student accommodation & care homes. Other premises should be considered if they are at greater risk of fire or difficulty of escape. Examples include, hospitals, health care facilities, cinemas, heritage buildings and thatch or timber framed buildings.
AFDD's are very expensive at the point of writing they were between £100 - £150 each so it is unlikely that there will be early uptake unless it is required.
Below is the circuit diagram showing the Micro controller unit.
IMD - Insulation Monitoring Devices
IMD's are designed to pre-warn the skilled person responsible for an electrical installation that the insulation is starting to show signs of degradation. The device monitors potential difference / current between live conductors and earth and when a threshold is reached a signal output is generated to prescribe a response.
The output can simply be a light or alarm or may be more technical and patch into a building management system that can automatically alert an engineer by phone. This in effect allows essential systems to be maintained before they breakdown. This can help service engineers plan work on similar systems based on the data that they collect. Applying collected data to a (PPM) Planned Preventative Maintenance schedule rather than relying on reactive maintenance reduces downtime and maintenance costs.
IEC 61557-8 details the requirements for IMD's which is supported by BSEN 61557. In BS7671 reference to insulation monitoring devices is made where an earth fault may not be sufficient to isolate the system. This method is intentionally used in Impedance earthing (IT Earthing systems) so IMD's are specified to alert the electrically instructed supervisor that a fault is developing before disconnection of the service.
IMD's are also specified when a system is supplied by an unearthed transformer for example if you are dealing with mobile or transportable electrical installations. Group 2 medical locations are required to be on an IT earthing system to prevent automatic disconnection of emergency life saving equipment. Group 2 locations include, radiology, operating theatres, intensive care, MRI and other areas where loss of supply may cause massive damage to sensitive devices or potential to cause danger / death. In medical locations a further requirement is to provide a function that raises the alarm if the connections to the IMD is lost.
Other examples where this level of protection may be prudent include:
Fire suppression systems in explosive or high hazard areas, coolant systems for reactors, railway signalling equipment, security systems, prison control systems etc. Note, the use of these devices requires a skilled operator to be available to maintain them at all times.
Residual Current Monitoring Devices
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Much like insulation monitors this device is to ascertain the current state of the electrical system. This device will monitor any current to earth and will initiate an alarm or signal if there is a fault. The device will not shut down the fault it is for monitoring and signalling g only. This may be used to determine when maintenance operations are due on critical electrical services. As per insulation monitoring devices this will be under the supervision of an electrically skilled supervisor.
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The one illustrated has two alarm states but some may offer a great many of different options. This may be incorporated into a building management system that can inform engineers of when and where faults are developing within a smart electrical installation. It has got to the point that the management system can phone an engineer to book servicing without the need for an intermediary person.
BS 1361 - Plug Top Fuse
The ubiquitous BS 1362 fuse provides an extra layer of overload protection that is not provided in most countries. The UK has some of the most robust regulations on earth. As a part of this we provide additional overload protection for individual appliances via a plug or switched fused connection unit (spur).
The additional benefit of this is the accessories socket outlets, connection units and plugs to BS 1363 are designed for 13A but are often protected by larger protective devices downstream that may allow them to be damaged in an overload. The fuse protects the fixed installation from individual items of equipment in fault and reduces disruption caused by circuit power loss.
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These fuses are 20 x 5mm and come in a range of standard ratings as shown in the table below.
The development of UK plug tops has led us to the modern types now that have an array of safety features. Firstly there is an Earth pin that is longer than the other pins. Its purpose is to make an earth connection first and additionally on a UK socket this will open the gate barrier that protects the live and neutral contacts from objects being poked into them. The live and neutral pins are insulated close to the plug body to prevent accidental contact when inserting or removing a plug. The arrangement of the pins makes it impossible to insert the plug in the wrong way without the use of a sturdy mallet! Finally it is becoming more common for appliances to be installed with moulded case plugs that are not serviceable by DIY dad, except for replacement of the fuse.
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Plugs should be marked to state they are ≤13A and built to BS 1363 they should also have a symbol of compliance with British Standards (BS) or Conformité Européene (CE) as shown
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​Moulded cases allow speedy replacement of fuses with little risk to the installer as shown below.
Selectivity
Discrimination / Selectivity & Cascading
When choosing protective devices it is not always a case of protecting an individual load. When you are dealing with distribution systems with a variety of loads it is a requirement that continuation of services is considered when selecting protective devices. Put simply I don't want my house to be in darkness because there was a fault in the shed!
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To achieve a good level of protection and continuation of service the means of protection should complement and act as back up systems downstream. A common misinterpretation of protective devices is that they are there only to protect the load. The true purpose is to protect the fixed wiring from excessive heat from current it was not designed to carry.
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To assist the designer in achieving acceptable levels of protection and selectivity manufacturers publish time current characteristic graphs that describe how the device will behave in conditions of over current or short circuit. The graphs will include any adjustable features or time delay functions of the protective device.
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The graphs can be used to select devices to achieve the least amount of disruption during a fault scenario. When you are using devices of the same type in general it is the case that to achieve selectivity the protective devices are lower rated values the further downstream you go. The example fault characteristics graph below demonstrates this.
The graph is on a logarithmic scale so you can use it effectively in one room!
To use the graph select the correct British Standard of the device and its type in this case BS 60898 type B. The most common protective device used in domestic properties.
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At the top pick the circuit breaker so for example the first curve is a B6.
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The Graph shows the amount of time the device would take to operate when it is carrying the current on the bottom of the graph.
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Any value to the right of line the device would disconnect or be destroyed.
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Where the line turns into a vertical it is showing instant disconnection.
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So in the case of the B6 instant disconnection occurs at 5x its rated current or 30A.
Time and current characteristics of individual circuit breakers and fuses can be found in appendix 3 of BS 7671 and published in manufacturers data sheets.
Time Delay Functions
Selective Devices
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There are a variety of devices that provide a time delay function to allow for downstream protective devices to operate. In some cases this function may have a variety of settings so good knowledge of the distribution system and expected loading is required.
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Another method of selectivity is reducing the protective devices as they get further from the source of supply providing their fault current and time graphs do not overlap which would indicate that they are not compatible.
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Below is an example of distribution that is representative of a domestic scenario with an outbuilding sharing the same supply system. The distributors fuse is the highest value device with circuit breakers and RCD's that are selected to allow the downstream devices to operate first. Even in this case it still does have the potential to open upstream devices if the fault current is too high but in simple overload scenarios discrimination should be achieved.
The suppliers fuse is a BS 88 HBC fuse rated at 100A.
Distribution circuit is protected by a 32A circuit breaker.
The final circuits in the out building are BS 60898 circuit breakers: 20A radial power circuit & 6A lighting.
The domestic circuits should be unaffected by faults in the outbuilding
As is demonstrated by the graph discrimination should be effective as the curves are not crossing each other.
RCD Selectivity
As well as selectivity in overload scenarios it can be achieved in earth fault conditions. A good example is in BS 7671 Electrical requirements for agricultural locations there are a few requirements for earth leakage protection.For fire protection 300mA RCD's are required.
Socket outlets > 32A a maximum of 100mA RCD is specified.
Socket outlet ≤ 32A are to be protected by 30mA RCD.
All other circuits including are to be protected by 300mA RCD.
By using this arrangement selectivity can be achieved in the case of earth leakage. Another method of achieving selectivity is to have time delayed RCD's that will have a short time delay in order to allow the downstream devices to operate. Time delays are commonly up to 1 second. BS 7671 & manufacturers instructions should be referenced when designing circuits with time delayed selectivity.