Fitting 5 tall RCBOs into a 12-way Volex CU

[tricia]

Hi

I have recently installed and wired up my 12-way split-load Volex Consumer Unit in,
my house, inserting 5 RCBOs and 5 MCBs (plus a door-bell transformer (2 slots.))
It all works fine but boy did it take ages to get the wiring tidy enough so that I could
fit in those damn tall RCBOs!

Is this a typical experience (for an electrician?)

When I bought the CU the shop (ALTO) told me that this CU will take the (single-slot) RCBOs and sure enough they fit in there fine, until you want all the wires in there too!
I am now wondering if I should have bought a CU that is _designed_ for {tall} RCBOs rather than a CU that merely will accept them.

Can one buy taller CUs that make life easier?

Fitting the MCBs was such a doddle, but the 5 RCBOs took about 10 hours solid to
make all the wiring inside very neat and hence fit!

Anyway, the CU is the one you see on Screwfix, a standard Volex 12-way split-load unit.
Would an MK or Wylex or Crabtree one be a better choice perhaps?

Any feedback (experiences/comments/opinions) would be much appreciated.

Thanks in advance,

Tricia
--

 

[plugwash]

if it was a split load why were you using so many rcbos?

 

[tricia]

if it was a split load why were you using so many rcbos?

Oh, I wanted ALL circuits to be RCD protected (overcautious of me perhaps) AND
I did not want a fault on one circuit (e.g. garage power) to bring down another circuit
(e.g. a lighting circuit or my kitchen ring-main (fridge-freezer)) So, the one big
RCD was not enough for me. That's why.

 

[plugwash]

why a split load in that case?

putting rcbos in the rcd side is pointless as you will likely trip both the rcbo and the rcd

 

[tricia]

why a split load in that case?

putting rcbos in the rcd side is pointless as you will likely trip both the rcbo and the rcd

Of course I didn't put the RCBOs in the RCD side, I put them on the unprotected side.

Anyway, what I'm after is answers/comments on my original _questions_ if anyone
has any.

Thanks

 

[plugwash]

id have said use the two module rcbos

also im pretty damn sure that all volex 1 module rcbos are type C which makes them a horror from anearth fault loop point of view

 

[ban-all-sheds]

Anyway, what I'm after is answers/comments on my original _questions_ if anyone
has any.

Thanks

OK.

Hi

I have recently installed and wired up my 12-way split-load Volex Consumer Unit in,
my house, inserting 5 RCBOs and 5 MCBs (plus a door-bell transformer (2 slots.))
It all works fine but boy did it take ages to get the wiring tidy enough so that I could
fit in those damn tall RCBOs!

Is this a typical experience (for an electrician?)

Not being an electrician, I can't say how typical an experience that is, other than to observe that whoever does it, they would have the same problems with space as you did, trying to fit tall RCBOs into a normal sized CU. They might be quicker at getting the wiring tidy, but the devices will be a tight fit for anyone.

When I bought the CU the shop (ALTO) told me that this CU will take the (single-slot) RCBOs and sure enough they fit in there fine, until you want all the wires in there too!

Did you ask them about getting the wires in as well?

I am now wondering if I should have bought a CU that is _designed_ for {tall} RCBOs rather than a CU that merely will accept them.

Bit late to wonder that...

Can one buy taller CUs that make life easier?

AFAIK, only in "industrial" ranges, not domestic, but I could be wrong. You probably didn't do yourself any favours by buying a C&N Volex in the first place.

Fitting the MCBs was such a doddle, but the 5 RCBOs took about 10 hours solid to
make all the wiring inside very neat and hence fit!

But think how much more satisfying it is to know that it's neat. Surely you wouldn't have done a rats-nest if you'd just been using MCBs would you?

Anyway, the CU is the one you see on Screwfix, a standard Volex 12-way split-load unit.

I note that the Volex RCBOs that Screwfix sell are the 2-module ones, not the tall ones....

Would an MK or Wylex or Crabtree one be a better choice perhaps?

Or Siemens, or Hager, or Merlin-Gerin, or...

You're not thinking of changing it just because it was difficult to do, now that it's done, are you?

Any feedback (experiences/comments/opinions) would be much appreciated.

I hope that's true

I'd like to add a question of my own - what circuits have you got on the RCBOs? If plugwash is right about them being Type Cs you shouldn't have any sockets on them or you'll not get the disconnection times you need. The fact that you say "I did not want a fault on one circuit (e.g. garage power) to bring down another circuit " makes me fear that you do have sockets on them....

 

[securespark]

Fitted an MK board with RCBO's a few months ago. Didn't notice any probs with space. MK board w@nky, but space no problem.

 

[tricia]

Can one buy taller CUs that make life easier?

AFAIK, only in "industrial" ranges, not domestic, but I could be wrong. You probably didn't do yourself any favours by buying a C&N Volex in the first place.

Oh well. What does C&N mean?

Any feedback (experiences/comments/opinions) would be much appreciated.

I hope that's true

Yes it is!

I'd like to add a question of my own - what circuits have you got on the RCBOs? If plugwash is right about them being Type Cs you shouldn't have any sockets on them or you'll not get the disconnection times you need. The fact that you say "I did not want a fault on one circuit (e.g. garage power) to bring down another circuit " makes me fear that you do have sockets on them....

Well, thank you for that question. I do have sockets on one circuit (kitchen ring main with
freezer on it.) Also, my garage power (lights and sockets) are on another RCBO.
The other 3 RCBOs are all lighting circuits.
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!

My system is a TN-C-S system (earth and neutral are connected together at
the service head as far as I can see)
So, are disconnection times a problem for me? Would not a Type B trip to often
on my freezer circuit?
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.

Thanks in advance.

 

[tricia]

id have said use the two module rcbos

OK, thanks for that.

also im pretty damn sure that all volex 1 module rcbos are type C which makes them a horror from an earth fault loop point of view

Please can you explain what you mean by horror and _why_ they are a horror?

Thanks.

 

[kai]

Type C MCB's are often considered taboo for domestic circuits, as they require very low earth loop resistance to disconnect withing the 0.4 secs in a earthed fault situation.

Type B MCB's are the normal ones to use in a household installation.

NEVER use a Type D in a household install - the D stands for Delayed Tripping, and your wiring would probably overheat or worse still catch fire, before it could trip.

 

[securespark]

Type C breakers require twice (approx) the current of type B to trip within 0.4s.

Type D require twice (approx) type C.


Let's clear up the use of breaker types - some seem confused:

According to the OSG:

Type 1 or B are designed for use in domestic and commercial installs having little or no switching surge.

Type 2/C/3 are designed for general use in commercial or industrial installs where the use of fluorescent lighting small motors etc can produce switching surges that would operate a type 1/B.
Type C/3 may be necessary in highly inductive circuits such as banks of fluorescent lighting.

Type 4/D are suitable for tranformers, X-ray machines and industrial welding equipment etc where high inrush currents may occur.

D type's instantaneous trip current level is 10-20 In, where In is the nominal rating of the device.

So, while D does not stand for delay, the level of current required to trip the device instantaneously is higher than type B (which is 3-5 In).

You can see that the basic difference between the breakers is their tolerance of inrush currents, simply put, how much surge on start-up they will tolerate.

 

[plugwash]

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

 

[plugwash]

I'd like to add a question of my own - what circuits have you got on the RCBOs? If plugwash is right about them being Type Cs you shouldn't have any sockets on them or you'll not get the disconnection times you need. The fact that you say "I did not want a fault on one circuit (e.g. garage power) to bring down another circuit " makes me fear that you do have sockets on them....

with type B mcbs and i think also type C though im unsure i don't think 0.4sec/5sec makes any difference to meet either you have to hit the fast trip and once you hit the fast trip they trip extremely fast

 

[ban-all-sheds]

Oh well. What does C&N mean?

Cheap'n'Nasty

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!

Reason says that there ought to be a Type A MCB, but I've never seen any info on one.

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. (Incidentally, what Kai says below isn't strictly true - it may be handy to remember that a type D is more delayed in its reaction, but there is no explicit time-delay built into type D breakers in the same way that there is in a type-S RCD. He's right about them having no place in your home, though. Unless you have an escalator in your home. But I digress).

The other way of looking at it is that type Ds need more current than type Cs, which need more than type Bs, to trip in a given amount of time.

In summary:
[code:1]Type Will not trip in Will trip in
100ms at rating 100ms at rating

B 3 x 5 x
C 5 x 10 x
D 10 x 20 x [/code:1]


I tried to find a graph showing the typical curves superimposed, but failed miserably, however if you go here: http://www.tlc-direct.co.uk/Book/3.6.4.htm you'll find a number of individual time-current curves that you can compare.

Why does this matter? One of the jobs of a protective device is to disconnect the supply in the event of an earth fault - i.e. if something goes wrong with your fan heater or toaster, and a live conductor makes contact with the earthed case, then a current will flow to earth. What we need is for that current to get large enough for the fuse to blow or the MCB to trip (the situation of current happily flowing to earth without tripping the breaker, and therefore with the case of the toaster remaining live is a Bad Thing™.) We also want the current to get large enough quickly enough such that the case doesn't remain live for very long, and the earth conductor doesn't have time to get hot enough to melt. This is called the disconnection time, and clearly, as I=V/R, we need a low resistance to get a high current. A C type breaker needs a higher I to trip quickly than a type B does, so a circuit protected by a type C needs a lower earth loop resistance than one protected by a type B.

How much lower depends on how fast you want it to disconnect. The wiring regs say that a socket circuit should have a disconnect time of 400ms. Lighting circuits, by contrast, require a 5s disconnect time,and there is a case to be made for using type C breakers on lighting circuits because they are much less prone to tripping with the brief current surge that occurs when a lamp fails than a type B is. At 5 seconds the curves for type B and C breakers have almost met, i.e. there is very little difference in the earth loop resistance limits for a circuit with that disconnection time.

Not so at 400ms though. The typical maximum values, in ohms, for the earth loop resistance (Zs), for 32A type B and C MCBs are:

[code:1]Type 0.4s 5s

B 1.50 1.71
C 0.75 1.6[/code:1]

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. )
For a TN-S supply, Ze is allowed to be 0.8 ohms, so type C breakers are flat-out not allowed for socket circuits, but even with TN-C-S, you can see that there's not much left for your R1+R2.

That, briefly , is what plugwash meant by "horror".

It's a bit of a mish-mash of links, but you can read more about this topic here:

http://uk.altavista.com/web/results?kgs=0&tlb=1&ienc=utf8&q=host:www.tlc-direct.co.uk+disconnection

Would not a Type B trip to often on my freezer circuit?

No.

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?