There are Two Types of Residual Current Device…
Residual current operated circuit breaker without integral overcurrent protection – an RCCB
Split load consumer units usually contain two RCCBs each protecting multiple circuits within the consumer unit with overload and short circuit protection provided via the individual circuit breakers.
Problems can arise when the RCCB within the consumer unit is ‘tripped’ as this will also cut off power to the other circuits being protected. This is known as nuisance tripping and can cause safety problems with a loss of lighting and defrosting of food etc.
The best way to avoid this although more expensive is through the use of individual RCD devices…
Residual current operated circuit-breaker with integral overcurrent protection – RCBO
A single device that combines the functions of a circuit-breaker (overload / short circuit protection) with that of an RCD (residual current protection) basically it’s a circuit-breaker with a built in RCD – “hence the price” and used to provide overload / short-circuit and residual current protection to an individual circuit.
RCDs have an integral test button. This will confirm that the device functions mechanically and a test should be done a regular basis, usually quarterly.
RCDs operate using the electrical principle that within any circuit – whatever current is flowing in the Line conductor, the Neutral will be the same
i.e. if the Line current is 2A then the Neutral current will also be 2A.
If there is a fault, say Line to Earth or Neutral to Earth then some of the current will flow through the Earth conductor and the current in the Line and Neutral will no longer be the same or ‘balanced’.
A residual current device will disconnect a circuit when it detects that the flow of current is not in balance between the line and the neutral conductor and that imbalance has reached a pre-set level / ‘trip current rating’.
As a shock current of only 50mA or higher is potentially lethal to the user of an installation 30mA is chosen as the rating for an RCD to protect the user from the risk of injury through an electric shock.
Other sizes are available of between 10 – 300 mA, but high trip current rating RCDs serve more as an additional fire-safety protection.
RCDS General…
They are designed to trip quickly and have a maximum disconnection time of 300 milli-seconds (which is 0.3 seconds) with a trip current of 1 x their rating. and 40 milli-seconds ( or 0.04 seconds) maximum disconnection time with a residual current at 5 x their trip rating.
An RCD on its own will not detect an overload or line to neutral short circuits and must be used along with an overcurrent protective device – as the current in the Line and Neutral conductor under these conditions will be the same.
i.e. 1000A in the Line and 1000A in the neutral – the RCD is happy but the conductor carrying the current isn’t!
RCDs should be used in…
- All circuits within a domestic installation.
- Any equipment likely to be used outside.
- Temporary installations and in places where there may be an increased risk of electric shock (special locations).
- And anywhere you cannot guarantee the integrity of the installation earthing / bonding.
- Portable generator supplies
Remember, fuses and circuit breakers provide short circuit and overload protection for individual circuits or equipment. 30 mA RCDs detect residual current and protect people from the risk of injury.