Fitting 5 tall RCBOs into a 12-way Volex CU

[pipme]

something im not to sure about maybe someone can clear this up

with a type C rcbo (which is all that wylex and volex make in 1 module)

1: do you have to worry about earth loop for the mcb part or does the rcd part make this irrelevent

2: if the rcd part makes earth loop irrelevent for the are there any rules on disconnecting phase-neutral shorts that need to be followed

Any comments

 

[ban-all-sheds]

something im not to sure about maybe someone can clear this up

with a type C rcbo (which is all that wylex and volex make in 1 module)

1: do you have to worry about earth loop for the mcb part or does the rcd part make this irrelevent

2: if the rcd part makes earth loop irrelevent for the are there any rules on disconnecting phase-neutral shorts that need to be followed

I'm afraid that yes, you do have to worry about the earth loop values when using RCBOs. At first sight it seems reasonable not to - an RCBO acting as an RCD will trip on faults to earth at much higher earth loop impedances than MCBs will. But tables 41B2 and 41C of the wiring regs list the same maximum Ze & R2 values for RCBOs as the underlying MCBs.

The reason is that in TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C RCBO may not provide a 0.4s disconnection time.

 

[plugwash]

ok having confirmed that bit

a type C mcb (and therefore the mcb part of a rcbo) needs 10x overcurrent to be sure of activating the magnetic trip (which is defiantely needed for meeting 0.4 sec and may even be needed for meeting 5sec i don't have the figures for type C breakers to hand

on a worst case TN-S it is impossible to meet disconnect times with a 32A type C and it could be hard to meet them even on a system with a much lower Ze (either TN-C-S or very good TN-S)

 

[tricia]

What does Type C mean then? Should I have used Type B or A then?
I think I need to know what a Type C one is!

...
B, C, and D have different time/current curves for tripping. Type B will trip faster than type C for a given overcurrent, and type C will be faster than type D...

As you have a TN-C-S supply, you might get away with type Cs on socket circuits, as the maximum allowable external component of the earth loop impedance (Ze) for that type of supply is 0.35 ohms. (And of course the DNOs strive ceaselessly to ensure that this limit is never breached.

Finally, my garage is about 50 metres away in cable length ( I will run a single radial
cable to the garage (armoured, 6mm^2 may be)) Is this OK? What would you recommend.

What load will it have to carry?

Well, thank you very much for all your answers and the links too!
That was very kind of you.

Regarding the load on my garage circuit, I expect to have 2 or 3 lights,
and use 1 or 2 power tools (e.g. drill say) so nothing excessive I would
say. So is 6mm^2 cable what I need or is 4mm^2 sufficient?
(I am mindful of the fact that I'll need to keep the impedance as low
as possible since I'll be using sockets on this radial circuit)

Finally, the big question really, will the Electricity Board most likely
PASS my installation when I call the man out to come and inspect
my house? (given the above discussion on my use of Type C circuit
breakers in my RCBOs)
How *strict*, as it were, are the testers/tests?
If you think it won't pass, what are the main things I should
concentrate on improving/altering?

Thanks.

 

[kai]

Rumour has it, the leccy board will pass almost anything connected to their system nowadays - the rules were relaxed in recent years - if you read some of these posts in detail you may see what I mean.

 

[fubar]

Finally, the big question really, will the Electricity Board most likely
PASS my installation when I call the man out to come and inspect
my house? (given the above discussion on my use of Type C circuit
breakers in my RCBOs)
How *strict*, as it were, are the testers/tests?
If you think it won't pass, what are the main things I should
concentrate on improving/altering?

The Electricity Safety, Quality and Continuity Regulations (ESQR) require that the installation complies with BS7671 and that an Installation Certificate be provided. Some regional electricity companies may be prepared to test and certify the installation before connecting it, but you will probably find it cheaper to get your local electrician to provide the certificate.

The NICEIC provide a list of the most common departures - you need to be familiar with BS 7671 (including electrical design considerations) to be able to evaluate whether your installation is likely to pass the inspection, although IEE guidance notes 1 and 3 will provide a good insight.

http://www.niceic.org.uk/nonapproved/departures.html

Hope this helps,
--
Michael

 

[ban-all-sheds]

Re the garage cable, if we assume, for argument's sake, a 15A max load, then 4mm² would be OK in terms of volt drop, but it is getting up towards the maximum allowed. I've never thought that deliberately designing in a large voltage drop, even if it's less than the limit, is good practice, so I'd suggest using 6mm².

In fact, if the cable is to be buried, I'd say use 10mm², just in case you ever decide to have a hot-tub out there, on the grounds of the marginal difference in the cable price compared to the cost and hassle of digging it all up again and laying in a bigger one.