Correct Installation?

[RyanM]

We had a contractor in at work, as no one here is qualified to install what we wanted, but is it correct?

We have a new machine coming, which uses 64A, so purchased a 125A interlocked switch.

The contractor has done the following:
Tap-Off-Box from Bus Bar, with 125A fuses -> 35mm^2 SWA cable -> 80A, 30mA Schneider RCCB -> 16mm^2 SWA cable -> 125A switch.

Is the RCCB correct though, as that isn't offering any overcurrent protection is it for the 16mm cable, so the only overcurrent protection is the 125A fuses in the top-off-box? Should there not be another MCB in there like an 80A one?


Thanks a lot

 

[ban-all-sheds]

Is the nature of the load such that overload protection is not required? Could be that 125A fuses provide adequate fault protection for 16mm².

Are distances such that 35mm² is needed for most of the length but the machine can't have that size connected to it?

Or was it done by a numpty who dropped from 35mm² to 16mm² because he thinks the RCCB provides an overcurrent function?

 

[RyanM]

I was thinking that the contractor maybe was thinking the RCCB will give the overcurrent protection?!

The distance from the tap-off-box to RCCB is approx. 5m (in 35mm^2), then from the RCCB to the 125A switch the distance is approx. 21m (in 16mm^2).

I didn't think the 16mm^2 cable would be protected by 125A fuses?

 

[ban-all-sheds]

I was thinking that the contractor maybe was thinking the RCCB will give the overcurrent protection?!

It's looking more likely.

I didn't think the 16mm^2 cable would be protected by 125A fuses?

It might be - I can't be rsed to do the sums...

 

[ericmark]

The regulations are as follows:-

433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation.
NOTE: A reduction in current-carrying capacity may be due to a change in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions.

433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type of cable or conductor. or in environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:
(i) It is protected against fault current in accordance with the requirements stated in Section 434
(ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons (see also Regulation 434.2.1).

433.3 Omission of devices for protection against overload
This regulation shall not be applied to installations situated in locations presenting a fire risk or risk of explosion or where the requirements for special installations and locations specify different conditions.

I have seen where isolators have been mistaken for MCB's (Loadmaster) and where RCD's are mistaken for RCBO's in one case RCBO's were ordered and RCD's arrived and no one questioned it.

I have also seen where a 16A MCB feeds a garage then in the garage a 32A MCB feeds a ring. And a 32A MCB feeding a ring of 16A sockets with non fused plugs.

However it is easy to jump to conclusions and get 2 + 2 = 5. In one place they had 15A sockets where the fuse was the line pin of the socket so they were fused correct. One guy shouting about a fuse in the neutral but it was a split phase supply without a neutral. So one does need to be careful when criticizing some ones work.

There are 63A RCBO's

but not very common so I would not expect to find one. At 32A and below there are loads but above that it tends to be an add on unit. The moulded breaker often has a separate box with amp and time controls and current transformer and in some cases the MCB and RCD do combine using the same contacts leaving what seems to be a blank and as a result it does not say RCBO on the part which the user can see.

 

[jeepmadmike]

I'm amazed how often I find RCDs or switch disconnectors installed when house bashers think they give over current protection

Also is RCD protection required?

 

[RF Lighting]

Yep I see it all the time. Favorites are showers and garage supplies.

As for the design in the OP, it all sounds a bit strange.

Why not fit 80A fuses in the tap off box and run 16mm² SWA all the way to the machine isolator?

As for something like that needing RCD protection, I find it highly unlikely.

I wonder if it's just another member of the 'I don't really understand the regs, but I think you have to RCD everythng now' brigade.

 

[Lectrician]

My thoughts too RF - 80amp fuses in the tap off, cable via iso to machine.

 

[ricicle]

Sounds to me like a domestic installer punching above his weight ?

 

[RF Lighting]

Certainly doesnt sound like someone familiar with commercial / industrial installation work.

 

[RyanM]

Hi,

Thank you very much for the replies.

Is a RCD needed? I don't know to be honest. My manager contracted this out so we didn't need to think about it, but out of my own interest I had a look at it to try to gain better knowledge for myself.
A RCD couldn't really hurt could it? The environment does get very wet (hygiene crew).

I also agree the easiest solution would have been 80A fuses in the tap off box. Although in cases when these could blow (water ingress?/other fault conditions), they aren't as convenient as a MCB to put back in, as it means opening up the tap off box etc. Although I suppose this is a minor factor if 80A fuses were actually blowing!


In this case then, is the cheapest solution for us to get the contractor to replace the 125A fuses for 80A?



Thanks a lot

 

[RyanM]

Also, could I possibly ask how to calculate the load of the 16mm cable please?

It is SWA, 21 meters in length, going through 100mm of fire proof ceiling, but also through 2.5m of stainless conduit.



Thanks a lot

 

[RyanM]

I've had a look in the 17th Edt book, and am I correct looking at Table 4E4A?

The SWA is 4 core, three phase. Method C would give a capacity of 94A, but then it does go through 100mm of insulation and steel conduit?

 

[RyanM]

Well the electrician came back, and fitted a MCB as well as the RCCB.

His excuse was he got confused....bless him.

 

[ban-all-sheds]

That's OK then - confusion in an electrician leading to inadequate circuit protection is fine.