What would you do if you got it wrong...

[mfarrow]

Hi

I'd just like to canvass some opinion as to the 'trades' resolution to a problem a friend's having with an electrician.

He had a flat re-wired and two boards installed - one for heating (E7) and another for the remaining circuits (24h). Both are protected by a 100KMF with 60A fuses inside.

After switching everything on, a day or two later the E7 fuse blew in the middle of the night. The electrician came back and put a clamp meter (he called it a 'load monitoring device' ) on the tails which read 78A. So he's agreed to do some remedial works.

His resolution is to put two of the storage heaters onto the 24h board and install a timer. Obviously this isn't an ideal solution on a new installation, but the electrician doesn't seem keen to replace the cable run from the KMF, which is 16mm T&E at present (he mentioned he couldn't terminate SWA successfully (agreed), but seemed to forget about the possibility of using meter tails!)

The run from the KMF to the E7 board is 8m through a bathroom (behind a removable panel) and a short lenth of ceiling void.

Do you think too much is being asked for here, or would you be happy with your installation if you left it the way the electrician is proposing?

 

[JohnD]

just to check, what is the purpose of the fused switch? Is there one per CU, or is it being used as a main switch for the whole installation? The 100KMF is fairly expensive.

I seems the calculation of design load for the storage heaters was either not done, or was incorrect, or has been exceeded by subsequent change - do you know which?

 

[mfarrow]

Is there one per CU, or is it being used as a main switch for the whole installation? The 100KMF is fairly expensive.

There is one per CU. The KMFs are Click sorry not MEM.

I seems the calculation of design load for the storage heaters was either not done, or was incorrect, or has been exceeded by subsequent change - do you know which?

AFAIK the electricians were aware of the size of the storage heaters, though they did not specify or fit them. The specification was done only once (by Dimplex). The place was gutted before they moved in so they would have had to asked.

 

[JohnD]

are you sure they are 100A switches not 60A which are similar but smaller and cheaper?

large size cable is rather expensive, he may have that in mind. Where are the RCDs?

 

[bernardgreen]

An alternative solution might be to power some of the heaters for the first half of the economy period and then switch them off and switch on the remaining heaters for the second half.

This would require delayed operation timer and ( depending on current loading ) a slave contactor with a pair of normally closed contacts and a pair of normally open contacts. Normally closed contacts feed power to one set of heaters, normally open contacts feed the other set.

Using two sets of contacts ( instead of a change over with common ) allows each heater to have its own fuse in the CU. This allows a heater to be isolated by its fuse if it faults without affecting another heater.


The timer would start to time when the Economy power was available and then after the preset time it would operate the contactor.

 

[mfarrow]

are you sure they are 100A switches not 60A which are similar but smaller and cheaper?

I'm pretty sure they're 100A, but even so I thought the next size down was 80A (which would be ideal)?

large size cable is rather expensive, he may have that in mind. Where are the RCDs?

The RCDs are in the consumer units. I think both boards each have 2 RCDs (i.e. a "17th edition board").

 

[ericmark]

I had a job where all the ratings for every item were supplied to me and like with your heaters they would be on all the time no diversity.
However the figures given were wrong and it used more than it said on the plans.
With that situation it was not my fault. And I had to consider my position on one hand I wanted to correct the fault but on the other hand I did not see why I should foot the bill for some one else's mistake.
Like your electrician I used a compromise.
Before you shout too loud find out who's fault. If all the heaters are marked with wattage it should all add up to less than 13800W or 13.8kw if it does then not the fault of the electrician.
However if the wattage exceeds 13.8kw then who ever designed the system has miss calculated and if he designed it then he should correct.
This assumes the heaters were there for him to read the plates before the design stage of the job.
Most heaters today are under 3Kw so they can be supplied with a 13A fuse. So for 60A one would expect to fit max of 5 heaters on that supply when they are 2.8kw. But with 2.2kw versions then 6 heaters could be fitted.
If you had for example 8 heaters fitted all at 2.2kw then clearly the guy simply can't add up and he should do all required to correct the mistake.
However you also have to consider the problem for you with disruption.
As to cable running singles is either restricted to 3 meters with double insulated or needs to be in trunking so really not an option for a long run. I have used 240mm 4 core flex so know flexible cables and other non wire armoured are available. But the cost is something else. So in order to physically get the cables in space then it may need two parallel cables. They must both follow same route and be same length but this was a common method to connect generators two 120mm cables are far easier to handle than one 240mm. However with 25mm it is not really an issue however it may that some time to order and would not be twin and earth.
Personally the problems of having to supply the cable with a RCD means I would have tried to use SWA so I could have the RCD in the consumer unit rather than at the DNO supply point. And why terminating SWA would be a problem I fail to see. With suitable shell under the SWA one can clamp onto the earth outer without use of a gland. In the same way as we do when making a cable joint.
So who made the mistake?

 

[ricicle]

16mm² t+e clipped direct = 85A

 

[JohnD]

mfarrow, you're right, the next size down (in MEM at least) is 80A

don't know why I said 60.

 

[mfarrow]

If all the heaters are marked with wattage it should all add up to less than 13800W or 13.8kw if it does then not the fault of the electrician.

The heaters are marked and add up to more than that, as that was how my friend added it up.

This assumes the heaters were there for him to read the plates before the design stage of the job.

They weren't there but had already been calculated.

Most heaters today are under 3Kw so they can be supplied with a 13A fuse. So for 60A one would expect to fit max of 5 heaters on that supply when they are 2.8kw.

There are 5 heaters, so more than 3kW each then I guess.

As to cable running singles is either restricted to 3 meters with double insulated or needs to be in trunking so really not an option for a long run.

I didn't think there was any restriction except when running straight from the meter?

I will suggest the flex idea, thanks.

 

[Lectrician]

16mm T+E would be fine on an 80amp fuse clipped direct....