4 pole isolator allowed?

[Christian McArdle]

Is it permissible to use a 4 pole isolator intended for 3P+N to isolate two single phase L+N supplies?

Christian.

[Andrew Gabriel]

In most cases, yes, unless the design of the poles is different for, e.g., an undersized neutral. Clearly, it will also have to be well labelled. What is the reason for not just using a single

2 pole isolator?

What you must not do is parallel up multiple contacts to make a higher current contact. You can parallel up contacts, but the total rating remains that of the lowest rated individual contact.

Also, bewhere of using 3-phase items for single phase -- you need to give extra thought to the suitability. The place I've seen electricians fall foul of this is using, say, a 3x100A

3-phase fuse box as a single phase 300A fuse box, and then wonder why it keeps burning out. On one occasion, I gave up trying to explain what was going wrong to the guy -- he was just never going to understand.

[Christian McArdle]

I wish to add a single point of isolation near my boiler/water cylinder. It would be good if it could isolate the L/N to the central heating and the L/N to the immersion heater simultaneously, as this is safer than having two indepedent switches. If it isn't allowed, I'd just have to use two switches, such as a twin 20A DP grid switch like the three gang one I used on my kitchen appliance radial circuit.

Although both circuits are on the same consumer unit way, they are installed in a loft and I wish to have the immersion switch in the kitchen without going up two floors to a single DP isolator shared with the CH than down to the switch and then back up again.

I believe there is a requirement to have either a lockable isolator, or an isolator near the equipment where maintenance personnel can guard against inadvertent activation. I would prefer a separate isolator in the loft. It is more convenient for maintenance as it is will not be necessary to go down the ladder and a flight of stairs to restore the power for testing.

Christian.

[Christian McArdle]

P.S. I had something like:

mind.

Christian.

[BigWallop]

Hi Christian,

Why not make your life easier by fitting a switched two gang socket and a couple of plug tops and when it comes to servicing the appliances, then all you do is switch off and pull the plug and know for sure that the appliances are well and truly isolated from the electrics.

[Christian McArdle]

That may lead to confusion. Although from the same circuit, one supply is off an FCU at 3A. The other is off a immersion timer fused at 13A that can't take inductive loads such as power tools which are the most likely alternative appliances to be inserted given the location.

Also, there's enough fuses in the system already. A few more will just complicate matters. If the 4P isolator isn't suitable, I'll just have 2 DP ones, clearly labelled.

Christian.

[Andy Wade]

If your're going to do that, why not just feed the CH off the immersion heater circuit (via an FCU fused at 3 or 5A)? With a standard immersion heater cct. (3kW load, 16A MCB) you've got 3 amps to spare, assuming that the cable installation conditions allow it. That way you'd only need a standard DP isolator.

I did precisely this at our last house, in conjunction with a home-brew control panel which allowed the CH programmer to control the immersion heater -- CO & centre-off toggle switch labelled "HW - GAS-OFF-ELEC". (The immersion heater should be switched via a contactor, BTW, not straight off the tiny relay in the programmer!)

One reason sometimes given for not doing this is that you'd lose both methods of heating water if the circuit failed or had to be isolated. But if you link the means of isolation for two otherwise independent circuits, you've demolished that argument anyway.

[Andy Wade]

But don't worry, his company will probably soon be signed up to a competent persons scheme for Part P and he'll happily be able to self-certify his expert work, helping to keep us meddling DIY-ers safe...

[Christian McArdle]

The immersion circuit will be switched by an immersion timer that is situated in the kitchen for convenience. This makes it unsuitable for use in supplying the central heating, as it will rarely be energised. I did consider using the main programmer and an electricgas switch, but decided that as it is a backup, I wanted to be able to isolate the central heating totally (including programmer) and still have water heating.

So, from the kitchen cupboard, where the consumer unit is, we have an immersion timer, with a 2.5mm cable running up to the loft. We have a 3A FCU, with a 1.5mm cable running up to the loft. This then goes into the wiring centre, where cables fan out to the programmer, zone valves, thermostats etc.

There are actually isolators at two points. (Apart from the MCB). There would be the 4P isolator in the loft, intended to isolate for reasons of servicing whilst up there. There are also separate immersion (via the timer) and central heating DP isolators in the kitchen cupboard, so it would be possible to totally isolate the central heating without affecting the immersion circuit.

Christian.

[Andy Dingley]

Because they're not appliances ?

If you disconnect a plug and socket, then you've also removed the earth. For some fixed equipment, rather than portable appliances, then this can be significant.

[Andy Wade]

[Christian McArdle]

Keep going though! It keeps me thinking about it before I make a c*ck up.

Christian.

[Tim..]

I have exactly the same arrangement! A simple 3 position (Boiler / Off / Immersion) switch plus contactor for the element allows the CH programmer to control the immersion heater in the summer when we turn off the boiler.

Tim..