Pendawaian / Wiring - Guidelines for good consumer unit design and low electric fields in the house

Consumer unit with RCB and MCBs

The consumer unit is the “fuse box” which goes between the electricity meter and all the electrical circuits in the house. Modern units do not contain fuses, but miniature circuit breakers (MCBs) which have replaced them.

Guidelines for good consumer unit design and low electric fields in the house (photo credit: jr-elec.co.uk)

They often also contain an extra protective device called a residual current device RCD, which replaces the isolator switch. The modern consumer unit can also contain other devices such as time switches and door-bell transformers.

Miniature circuit breaker (MCB) 16A B 1p – type ACTI 9 Schneider Electric

I also recommend the use of screened wire for the same reason in order to minimise electric fields in the consumer unit.

The purpose of the MCB is to protect the wiring of its circuit from excessive current due to an overload or short circuit – in a similar but more controlled way than a wire fuse did. If the rated current is exceeded, the MCB will trip, and merely needs to be switched back on again to reconnect.

It is more convenient than a fuse, and it is more obvious to see which one has tripped after a fault occurs. Common values are:

• 6A or 10A for lighting circuits,
• 16A or 20A for radial power socket circuits and immersion heaters,
• 32A for ring mains, 7kW electric showers and electric cookers, and
• 40A or 50A for 8kW and larger ones.

 

Residual current device (RCD) – 4P 40 A 0.03 A 230 V/AC – ABB

RCDs (sometimes wrongly called Earth Leakage Circuit Breakers – see above) are primarily designed to protect against electrocution (death from electric shock) by detecting unbalanced current flows. The RCD monitors the balance of the Neutral and Phase currents and disconnects the circuit if any out-of- balance current reaches a pre-set limit.

Because under normal circumstances there should be very little earth current, they can be made very sensitive: 30 milliamps (0.03 A) is common, with 100 mA (0.1 A) for circuits with motors and higher current switched loads.

In some cases, the RCD does not protect all the circuits, in which case there is also an overall isolator switch to disconnect everything. This type of arrangement is called a “split load” consumer unit.

 

Under the latest  UK Wiring Regulations (BS 7671) many more circuits are required to be protected by a 30 mA RCD and some consumer boxes now have two split multi-MCB circuit groups each of which is protected by an RCD.

 

It is normal to feed a freezer from an unprotected circuit so that in the event of a trip while the house is unoccupied, it will continue to function. Garages and outhouses are also often fed in this way if they have their own RCD, so that the main house RCD will not trip in the event of a fault.

IMPORTANT! RCDs do not protect against overloads so must always be used in conjunction with MCBs.

An RCD usually protects several circuits, any one developing a fault will result in them all being disconnected. It can be better to use an RCBO (a combined MCB and RCB) for each circuit, though these are significantly more expensive and physically larger at the present time.

Some advanced RCDs and RCBOs have extra features, such as detection of live/neutral reversal or earth disconnection (for which they need an earth sense wire).

Confusion over RCD and ELCB

RCDs are current operated, but there is also an older voltage operated Earth Leakage Circuit Breaker (ELCB). There is some confusion over these names. They sense the voltage between the earth of the protected area (e.g. all the sockets and pipes in the house) and “true” earth (the sheath of the incoming supply cable or an earth rod).

When there is sufficient leakage current the ELCB will trip, disconnecting the circuits protected by it. It then has to be reset manually.

Voltage operated devices have the disadvantage that they can only detect leakage to the earth circuit that passes through them. Thus, they will not detect current passing through the body to the ground when using equipment outdoors, or to current passing through the body to plumbing if it is connected to the ground and therefore cannot be included in the protection circuit.

They are now only used when the house is supplied by what is known as a “TT” system where a Protective Earth Conductor is not supplied by the electricity supply company and the house relies on a local Earth Electrode buried in the ground. Even where TT systems are used, an RCD is generally now preferred to an ELCB.

Here is a more detailed diagram of a traditional UK final ring circuit:

Detailed diagram of a traditional UK final ring circuit

An alternative for electrical designers and electricians who insist on “traditional ring final circuits” is the “folded ring”.

Two cables are taken together around the rooms, one connecting to the socket outlets and the other connecting to the final socket outlet to complete the loop. This ensures that magnetic fields are minimised.

Radial circuits have low emissions of magnetic fields. It is often possible to convert an existing “ring” final circuit into one long radial circuit without major rewiring by removing one end connection and lowering the value of the protective device to 20 amps.

 

In other cases it is possible to break the “ring” near its middle and, likewise, protect the two new circuits with either one or two 20 amp circuit breakers depending on the expected electrical loading.
2 x 20A radial circuits provides for more load than the original ring final circuit.

Ring final circuit split into two radial final circuits

If a Residual Current protective Device (RCD) is used, any significant imbalance in Line and Neutral currents (i.e. any Net currents, which will almost always raise magnetic field levels) will cause the circuit to trip out and indicate a fault. RCDs are usually double-pole devices that switch both the “line” and “neutral” conductors.

They can cause unnecessary circuit trips when some high loads are suddenly applied to the circuit, especially if these are highly inductive or capacitive loads.

An RCD can be combined with an overcurrent protective device (MCB) in one unit and it is then known as an RCBO. It is now a legal requirement that any (newly wired) sockets that might be connected to an extension cable to be used outside (i.e. if they are close to a garden window) must be protected by and RCD or RCBO.

 

An earthed metal conduit system (with cables or insulated wires in metal pipes), with radial final circuits will always produce the lowest electric and magnetic fields. The earthed metal pipes completely screen the electric field but, generally, have little effect on the magnetic field.
Another common cause of high magnetic fields comes from poorly laid-out lighting wiring. When possible, Phase (Live) and Neutral conductors should always be run together, keeping the “go” and “return” currents together in the same cables.

To minimise magnetic fields, both Live and Neutral should be taken to each luminaire (light) and then the Live take to-and-from each switch as a twin and earth cable! The Live and Neutral conductors should always be connected to the same circuit from the consumer unit box and should never be connected between different circuits.

It is not uncommon to find two-way switched lighting circuits that, incorrectly, interconnect different circuits. This is against the requirements of BS7671. It is important that such circuits are fed from only one Live and are kept with their own Neutral; special 3 core + earth cable (easily available) is required to do this satisfactorily.

Reference // House Wiring and EMFs by Alasdair and Jean Philips