UK Dis board position change and upgrade, cable compliance issues

[John-]

Hi, I think I may have helped hold the gun in the shooting of my own foot…

Been combining TP dis boards into a single board, after a workshop extension. Quite a few 4 core 1.5mm SWAs feeding fixed equipment, like TP drills lathes and the like – all fed via 16A breakers on the old board.

Lots of the cables were just slung in purlins, that are no longer there due to the extension, so I had to extend them and move them onto a new Cable trays.

Just took a look at the regs re cable sizes and I see that in putting in nice new cable trays and fixing the cables to them, I have now introduced a grouping factor (to be fair it was already there so I think I have inherited this). The It value for 1.5mm SWA (4D4A) is 19A. Grouped, now moves this to around 24A…

How serious is this as an issue, considering its been in for over 20 years like this…

Options to put right I guess are, to reconfirm circuit Ibs (which i should have done to begin with) and reduce IN if I can. Failing that increase cable to size for the correct rating, 2.5 / 4mm.


Any ideas?

Cheers
John

 

[pc1966]

Any idea of the duty cycle on the machinery?

It is it 1-2 folk using 4+ machines then it is very unlikely they will all be loaded at the same time, which is what the grouping factor assumes.

For many situations for an EICR they mention evidence of thermal stress, so if the cables have been fine for the last couple of decades it is probably not a significant issue now.

Are there any issues with the circuits failing to meet the Zs for disconnection under earth fault?

 

[John-]

Hi

It is 1 person that uses 1 machine at a time, maybe 2 people will use 2 of the machines at the same time, for just a very short period of time. I see what you are getting at, the thermal conditions of grouping only occur when the surrounding cables are in use contributing top the build up of heat, I never really looked at that way... I always just took it as the insulating impact of the surrounding cables, with or without current flow (knob). But you are right, makes sense - smarter every day...

No evidence at all of thermal stress. I will be doing a 100% test and inspect of everything. Found all sorts of minor issues and fixed as i go - its day work fortunately.

No issues at all, so far with Zs - touch wood...

Thank you.

John

 

[John-]

The thing that sparked this off was the request from the customer to install a 16A TPN socket at each of the new Roller Shutter Door (RSDs )positions using the proposed new cables I have put in, for the RSDs.

At the moment the RSDs are fed via TPN sockets on a temporary extension lead (supplied by the RSD suppliers). He now likes the idea of having TPN sockets next to his doors. I had planned to remove the plugs and fit an isolator, I have already installed 1.5mm 4 core SWA, subject to same grouping conditions fed via 16A MCB for the doors - just not yet 2nd fixed them. But looking again i see no reason why i cannot fit the isolator, leave the plug socket arrangement and instead of the MCB, install a 16A RCBO in the board for each, so that if he wants to use a separate TPN portable device, all he has to do is operate local isolation, unplug the RSD and then plug in the portable appliance. I am guessing it will be a pressure washer.

I originally said no (power via a 1.5mm cable, don't be daft), but given the above example i am not so sure. Again the duty will be low from surrounding cables in that area and it will get used as a TPN socket once a month if you are lucky.

One run is about 7m, the other is about 17m away for the board - both pretty short runs. The difference here though is the IB is always set at 16A - I just don't know what he will plug in.

A stretch too far?

Cheers

John

 

[telectrix]

if his equipment is fitted with a 16A plug, then it won't draw more than 16A.

 

[Wilko]

Sorry if I’ve misunderstood, but if the circuits were protected by 16A rcbo doesn’t that fix the grouping factor and unknown plug in loads?

 

[John-]

Sorry if I’ve misunderstood, but if the circuits were protected by 16A rcbo doesn’t that fix the grouping factor and unknown plug in loads?

Hi. Circuits were just going to be MCCB (they are new), they are still subject to the same grouping issues as the previous cables. I am proposing them to be dual purpose now, one for door and one socket. The RCBO comes now i am using it as a socket... It just does not sound right putting a socket on a 1.5mm cable .

 

[telectrix]

for a 16A load anything bigger than 1.5mm is overkill (assuming no severe derating of the cable is applied).