RCBO

[safesinbad]

Hi there

Something that i've been racking brains for is why is there a cpc connected to an RCBO. I have been told it is for functional purposes but why, an RCCB doesn't have one and trips when T is pushed. When i left the cpc disconnected and pushed T on the RCBO it also tripped, i connected it back to earth tho. If anybody does know why please let me know, i know its not essential to know why im just curios.

thanks

 

[Spark123]

The lead you speak of is a functional earth. These are often found on RCDs which have elecronics.
There are a couple of different reasons from different manufacturers I can think that they are used for - MEM use the funtional earth to also monitor Line to Earth voltage. I believe Square D also look for these so if the earth potential was to rise the device trips.
Another possible reason is when there is a problem on a circuit and there is not enough volts to drive the electronics within the device, the connection to earth should have enough volts between it and line to cause the device to open.

 

[safesinbad]

cheers mate, sounds obvious now lol

 

[NotHimAgain]

RCDs have been around since the 1920's. The big break through was an increase in sensitivity achieved by the use of a 'precision' relay.

The basic single phase RCD used to consist of a toroidal core with line and neutral windings on it, and a detection coil. Any imbalance between the magnetic fluxes produced by line and neutral coils would produce a small resultant flux and the detection coil would pick this up and directly drive a 'precision' relay. This relay coil can be operated by a relatively low energy input and it can be arranged to trigger at a set threshold.

This is still how some RCDs work - hence most of the double pole devices don't have functional earths.

Now anything that has the word precision in it tends to be expensive to make. The 'precision' relay falls foul of this. The size of the relay is also an issue if space is limited. Note that this arrangement does not require any energy input other than that provided by the detection coil.

Enter the 'active' RCD - this replaces the precision relay with a difference amplifier. These devices can have high sensitivity, and can be made very cheaply. They can also be relatively small. Ideal then for the limited space available in single module RCBOs.

The downside is that this difference amplifier needs a power supply. Two things could remove this: loss of neutral and supply voltage collapse.

Loss of neutral is resolved by the functional earth which simply provides a line to earth circuit for the amplifier supply.

Voltage collapse can occur in TN systems (not TT) when high fault currents flow. Basically the fault current causes the voltage at the installation to reduce due to the voltage drop in the supply network.

If the voltage disappears the RCD would have no power for its amplifier. The standard (BS EN 61009) specifies performance requirements and also requires that the RCD should open automatically below a stated voltage regardless of fault condition.

The good people on JPEL/64 (the Wiring Regulations committee) are so confident about this (not!) that they have included RCBOs in table 41.3. In other word they are saying - yes we know its an RCD - but make it work as an mcb if you can .

This is what we are now trusting to save our lives