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Anatomy of a consumer unit

websitebuilder • 2 June 2022

Anatomy of a consumer Unit

Consumer Unit – The ‘Fuse Box’ - Board. What's inside?


Never remove the front of your consumer unit.

DANGER TO LIFE

There is no need for normal operation.


The hinged lid can be opened to allow access to the devices/switches.


Before the consumer unit:


The below can be found inside a meter box outside the property or inside near the consumer unit.


DNO cutout & Fuse The Distribution Network Operator (UK Power Networks in the South East) provide the power to your home and they terminate it as a cut out which has a fuse (60 - 100 Amps). Provided, owned, installed and maintained by the DNO. Not your property.


Meter: connected between the main cutout and the consumer unit. Provided, owned, installed and maintained by the electricity supplier (who you pay the bill too). Not your property.


Meter tails - Large cables coming into the consumer unit from the meter. Generally either 16mm² (60 - 80A main fuse) or 25mm² (80 - 100Amp main fuse).


Isolator - double pole switch (switches both live and neutral). Rated at or above the cutout fuse rating. Allows the power to be isolated after the meter (without disconnecting the supply to the house).


The meter is connected with meter tails to the consumer unit main switch. Everything from this point is subject to the wiring regulations.


Wiring Regulations. BS7671 (IET Wiring regulations), currently the 18th Edition. This edition came into law on 1st January 2019. Amendments are made throughout an edition. Amendment 2 was published in 2022 and replaces Amd1 in Sept 2022.


Main Switch - the master switch that will turn off all circuits connected from it. Operating the main switch should be avoided except in an emergency. It is better to switch off each individual circuit first and the Main Switch last. The main switch will have a current rated this should be the same or higher than your main fuse.


Bus bar - a large copper bar that connects the main switch to the protective devices.


RCCB - Residual Current Circuit Breaker without integral overcurrent protection - The RCD protection only. This is normally connected to the main switch and supply power to overload protection (MCBs). RCCB's have a test button and should be tested every 6 months.


RCD – Residual Current Device. Provides additional protection when there is earth fault. The 18th Edition of the BS7671 wiring regulations require new or modified circuits to be protected by an RCD (now includes lighting circuits). The device measures current flowing in against current flowing out - when they match the magnetic coil is balanced and cancels each other out. A difference will unbalance the coil and operate the device shutting off the supply. Typically, the RCD will operate at 30mA - 30 thousandth of one amp. RCD's have a test button and should be tested every 6 months.


RCD – Types

Type AC – protects against AC only – widely used for several decades but affected by the wide use of electronic equipment that introduce a DC leakage current that can prevent it from operating. Old technology, no longer fitted as standard and no longer recommended in BS7671.


Type A protects against AC and DC current (< 6mA). The Type A is recommended where electronic equipment is in use anywhere on the circuit. TV’s, computers, USB charging and induction hobs. This is because this DC leakage current can saturate the AC type and stop it from operating.


Type B, F (EV charger and Solar Panel types).


MCB – Miniature Circuit Breaker – protects on overload current only. MCB's have different current curves (the speed in which they will operate for a current. The curve type is shown by the current rating. A B32 is a B curve 32A mcb.


Split Load Board - One or two RCCB's protecting different but multiple circuits. One may protect only sockets, the other lighting. Previous versions of BS7671 did not require lighting to be protected and these boards will only have one RCCB. This layout is no longer recommended due to larger amounts of leakage current that can cause nuisance tripping, replaced by RCBO. Where 2 RCCBs are fitted this was referred to as 'High Integrity' to differentiate the two types.


RCBO Residual Current operated circuit Breaker with integral Overcurrent protection. In essence an RCD and MCB combined. RCBO's have a test button and should be tested every 6 months.


SPD - Surge Protection Device. Protects the installation from overvoltage (surges). These were a new recommendation in the 18th Edition amendment 1 but now required in amendment 2. SPD's have a window which changes colour when the module needs replacing, surges wear it out over time.


AFDD – Arc Fault Detection Device. Software within the device monitors the sinusoidal wave form and are able to detect an arc signature. This device can trip when cables are damaged, short circuited or suffer say rodent damage. These devices will generally also include the mcb and RCD electronics. Currently veryexpensive but mandated in the amendment 2 to BS 7671 (2022). AFDD's have a test button (for the RCD) and should be tested every 6 months. They self test and have a sequence of flashing lights to determine why they operated.


If you have any questions about this please send me a message through the contact page.


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