Type B / C

[Albert]

Table 41.1 does not say what has to disconnect within times shown and RCD will disconnect in under those times. See Note 2.
On my reading the only place where you can't used a RCD to comply with times is with a TN-C system which we can't use anyway.

I just wanted to mention that RCD on its own is not enough as a protective device and they in principle protect against different faults than MCBs. So if you have RCD and MCB, the standard RCD should trip in max 40ms for x5 (although x1 is still required in the certificates, it is not required by BS7671), and not more than Idn = 30mA.

 

[ericmark]

I just wanted to mention that RCD on its own is not enough as a protective device and they in principle protect against different faults than MCBs. So if you have RCD and MCB, the standard RCD should trip in max 40ms for x5 (although x1 is still required in the certificates, it is not required by BS7671), and not more than Idn = 30mA.

All I ask is where in the regulations does it say a RCD is not enough? If we consider the RCD as being 150ma i.e. 5 x 30ma this is able to be tripped with a Zs of 1500 ohm.

Regulation 411.4.5
Zs x la ≤ Uo
where:
Zs is the impedance in ohms (&#937 of the fault loop comprising:
Ia is the current in amperes (A) causing the automatic operation of the disconnecting device within the time
specified in Table 41.1
Do remember this is one of the items which has been up-dated See here for BS 7671:2008, Corrigendum (July 2008)

 

[Spark123]

I think you are starting to get your wires crossed, an RCCB needs to be used with an overcurrent protective device as on its own, an RCCB does not afford overcurrent/L-N fault current protection.
An RCBO on the other hand does afford protection against overcurrent and fault current.

 

[ericmark]

Sorry I am not saying you don't need a MCB what I am saying is where a RCD is used you don't require the fast acting magnet part of the MCB as the requirement for fast acting disconnection is covered by the RCD.

As to neutral / line faults requiring the magnet part there may be a system which does require this. But at the moment I can't bring one to mind.

normally if one required super fast disconnection for short circuit one uses a fuse. Common with semi-conductor control.

I do note no one has found a regulation to show I am wrong. I was caught out also when I said you needed the B rated MCB and I also hunted to show the guy he was wrong but could not find anything to show the magnet part was required.

My thought was why would the manufactures make a RCBO with a B or C rating if not required so I was sure I would find a regulation but no.

 

[Albert]

All I ask is where in the regulations does it say a RCD is not enough? If we consider the RCD as being 150ma i.e. 5 x 30ma this is able to be tripped with a Zs of 1500 ohm.

I do not have the time to search for the reg mentioning that the RCD is not enough, but I can promise you, and this appears several times, that the RCD is considered as additional protection, additional to what? If you look at the book you will find it. When I will have a moment, if you did not find it yet, i will search for this.