Storage heater wiring at consumer unit

[jbenny1]

Hi all,

I've come across a bit of a problem in a house that i've never seen before, it has all electric heating, 10 storage heaters and 2 imersion heaters in the hot water tank. My problem is one of the existing 30A fuse holders that feeds 2 of the storage rads has overheated and melted part of the comsumer unit, i can't find any faults on the storage heaters so i come to the conclusion that as the consumer units are getting on for 35 years old the contacts have worn and lead to a poor connection which caused the overheating.

The set up is a little unusual as there are 3 meters On peak (Day), Economy 7 (Night) & Economy 10 (the old Off Peak tarrif)

The normal house wiring, sockets, cooker, lighting ect are all fed from the main consumer unit with RCD's and in turn feed from the day and night tarrif meter. The storage heaters are fed from 4x 2 way consumer units which are feed from the Off Peak meter, but some of them share fuses, ie. 2 or even 3 heaters on a single 30A fuse and a single radial curcuit in 2.5 or 4.0 twin and earth cable, which i assume was ok all those years ago.

I only need to change one of the 2 way offpeak consumer units, however i'd feel better changing them all as one has failed the others will proberbly go soon as well. I was just going to do a direct swap for a 2way garage style consumer unit 63A main switch, and 2 32A RCD's to replace the 2 30A fuses. However to tidy the instalation up i could fit one large 13Way consumer unit feed from the offpeak meter, however the total load on the main switch wold be 208A ! I have found a dual tarrif Wylex unit with 2x 100A main switches which would be better.

Also i have traced the feed cables from the offpeak meter to the 4x 2way consumer units and they all pass through a 75A 3 phase Heating Contactor, which has the 200A offpeak load diversified across the 3 phases.

Any ideas on where i stand, i know the latest edition of wiring regs say that all the storage heates should be feed on a single radial curcuit protected by 16A RCD, this would require me to completly rewiring a 5 bedroom house to get the cabling correct, or can i just swap like for like replaceing 30A fuses with 32A breakers?

Any suggestion much appriciated.

 

[flameport]

Impossible to know exactly what is going on there without seeing it.
However replacing whatever is there with 'garage style' consumer units is wrong. So are the other things you suggested doing.

What is clear is that the scope of this work is far beyond any DIY person's capabilities.

 

[Wontdothatagain]

Any suggestion much appriciated.

Consult a qualified electrician

 

[jbenny1]

Sorry for the confusion in my post i meant to say replace 30A fuses with 32A MCB's, my poor typing.

I've done a bit more digging around and following cables. It looks like a three phase heating contactor (Heavy duty high Amperage relay switch) was used to try and split the load down from nearly 200A single phase, to a far more manageable 60-70A per phase. The individual phases are then combined and connected to the 0ff peak meter. And the same reason the storage heaters are spread over 4 2 way consumer units is to split the load on the main switch, to prolong the life of the components.

Speaking to MK and a few other manufacturers, a 100 main incoming switch on a consumer unit can safely carry 100+ Amps but it is not designed to pull more than about 60A for more than an hour otherwise thermal cycling effects can cause the conductors inside the components to breakdown and increase resistance, this will lead to higher resistance and faster detrition of the components, leading to premature failure. MK recommended either using the 4 2way consumer units as per the existing installation, or alternatively a large scale industrial distribution panel with a 3 phase 100A main switch (giving 300A total capacity) and splitting the load across the 3 phases on the main switch within the panel.

Unlike most household curcuits which will never run at more than 50% capacity for more than a few minutes, hence the 100 square meter rule as per the IET Wiring Regulations (BS 7671). Storage heaters will pull their full capacity 2kw, 3kw or even 3.2KW for the full 8 hours of the cheep electric tariff, creating a whole new set of problems for the electrician to consider. This is why most electricians will not touch a house with storage heaters as there is a lot to take into consideration and every connection has to be very very well made due to the high loads and durations encountered.

As the existing setup has functioned fine for the past 30 or so years i will stick with this, it will also mean less upheaval in the meter cupboard as i will be replacing like for like i won't need to lenghten any wiring introducing more joints which is not really desirable.

 

[AndyPRK]

don't forget the heaters do have thermostats in them, but yes the first few hours they will be on solid.

If you really do have "2 or even 3 heaters on a single 30A fuse and a single radial curcuit in 2.5 or 4.0 twin and earth cable" I think you do need to check it out. And could well explain the problem with the 30A fuse holder.

 

[flameport]

I've done a bit more digging around and following cables. It looks like a three phase heating contactor (Heavy duty high Amperage relay switch) was used to try and split the load down from nearly 200A single phase, to a far more manageable 60-70A per phase. The individual phases are then combined and connected to the 0ff peak meter.

No, totally wrong.
It's a three phase contactor because it's a three phase supply. Nothing is 'combined' or 'split' anywhere.

And the same reason the storage heaters are spread over 4 2 way consumer units is to split the load on the main switch, to prolong the life of the components.

Rather more likely is that the person installing it had no clue what they were doing, or had a load of old 2 way consumer units they wanted to get rid of.
The fact is has been there for years doesn't make it correct, safe or suitable for use either.

Speaking to MK and a few other manufacturers, a 100 main incoming switch on a consumer unit can safely carry 100+ Amps but it is not designed to pull more than about 60A for more than an hour otherwise thermal cycling effects can cause the conductors inside the components to breakdown and increase resistance, this will lead to higher resistance and faster detrition of the components, leading to premature failure. MK recommended either using the 4 2way consumer units as per the existing installation, or alternatively a large scale industrial distribution panel with a 3 phase 100A main switch (giving 300A total capacity) and splitting the load across the 3 phases on the main switch within the panel.

That is what you get when a grain of truth is mangled by those who have no idea what they are talking about.

Unlike most household curcuits which will never run at more than 50% capacity for more than a few minutes, hence the 100 square meter rule as per the IET Wiring Regulations (BS 7671). Storage heaters will pull their full capacity 2kw, 3kw or even 3.2KW for the full 8 hours of the cheep electric tariff, creating a whole new set of problems for the electrician to consider. This is why most electricians will not touch a house with storage heaters as there is a lot to take into consideration and every connection has to be very very well made due to the high loads and durations encountered.

Mostly incorrect. Plenty of circuits are used at well above 50% capacity for extended periods. There is no '100 square meter rule'. Most electricians are perfectly capable of installing storage heaters and designing suitable circuits for them. The issues in designing and installing circuits for storage heating are exactly the same as those for any other circuit.

As the existing setup has functioned fine for the past 30 or so years i will stick with this, it will also mean less upheaval in the meter cupboard as i will be replacing like for like i won't need to lenghten any wiring introducing more joints which is not really desirable.

If the fuse has burnt/melted, the cable will also have overheated and therefore will need to be replaced.
There is the obvious issue of how you intend to disconnect the supply to replace these consumer units.

It's fairly obvious you will just do whatever you want, regardless of any comments provided here.
This is notifiable work, so carry on and break the law. Hopefully you won't kill anyone.