RCBO Type

[sparkticus]

Problem solved now... I have managed to find some type B ones. Thanks for all your help guys

Good, that is the right solution. But why do you need to fit an RCBO anyway?

The board in question is a 24 way three phase one and it supplies sockets to a kitchen area, coffee shop area and lots more. The socket outlets need to be protected right....

Well, they were protected by the MCB for overload, short circuits and earth faults. With the RCBO you have provided additional protection. I was just wondering what has triggered that need? I am assuming this was an existing circuit that had previously been certified?

 

[sparkticus]

This is something I have questioned. Looking at 442.5 it seems we may need to have a 5 seconds disconnection time but that does not seem to be cut and dried.

erickmark, 442.5 only relates to over voltage during a phase-neutral short. Obviously that is a reason to disconnect but in my mind over current (due to the short) is a more or at least equally pressing matter is it not?

I understood that 5s disconnection is only applicable to distribution circuits (sub-mains for example) or final circuits above 32 amps. No wait, maybe that includes 32 amps?

 

[steviez]

Problem solved now... I have managed to find some type B ones. Thanks for all your help guys

Good, that is the right solution. But why do you need to fit an RCBO anyway?

The board in question is a 24 way three phase one and it supplies sockets to a kitchen area, coffee shop area and lots more. The socket outlets need to be protected right....

Well, they were protected by the MCB for overload, short circuits and earth faults. With the RCBO you have provided additional protection. I was just wondering what has triggered that need? I am assuming this was an existing circuit that had previously been certified?

It all has previously been certified but years ago. Thought it best to give them more protection seen as there is no RCD on site anywhere.

 

[mfarrow]

This is something I have questioned. Looking at 442.5 it seems we may need to have a 5 seconds disconnection time but that does not seem to be cut and dried.

Exactly, ericmark!

The only thing I can think of, logically, is that the regs have assumed that the L-N impedance will be as good, if not better in most cases, than a L-E impedance. What maybe they've forgotton to do, is take into account the use of RCDs for the purpose of L-E?

Or... the definition of 'Short-circuit current' is "An overcurrent resulting from a fault of negligable impedance". So, at what point does that impedance become negligable?

I can't explain Table 43.1, but I don't think we should be reading into it too much as it's only information - I'm sure there are bigger tables for longer disconnection times elsewhere.

 

[sparkticus]

The only thing I can think of, logically, is that the regs have assumed that the L-N impedance will be as good, if not better in most cases, than a L-E impedance.

That should always be the case. In this case (a ring final) we would check end to end resistance of all conductors and although we don't record it on the cert that includes rN as well as r1 & r2. We then go on to measure R1+R2 by cross connecting right? Obviously the same would be true of a radial, we would test the neutral continuity/resistance.

Or... the definition of 'Short-circuit current' is "An overcurrent resulting from a fault of negligable impedance". So, at what point does that impedance become negligable?

When it is equal to or lower than the max Zs permitted for the circuit protective device by BS7671. I say that because if Zs is equal to or lower than the max permitted then Z(r1-rN) should be equal or less than Zs.