Economy 7 supply wiring question

[Roger Hayter]

I have an economy seven supply. Two 25mm wires go from the main fuse block to the meter (presumably live and neutral) and then to a switch and then to two Henley blocks. From one of these a 25mm wire goes to a radio controlled switch, and out of this time switch another 25mm wire goes to a third Henley block. The house supply runs from the first two blocks, and a separate wire from the third block presumably supplies live to the night time only supply.

However, there are two 10mm wires from the main fuse block to the radio switch, and one 6mm wire joins the radio switch to the meter.

Can I assume that none of the Henley blocks is live when the main fuse is open? In theory any one of the last three wires by-passes the main switch and could internally connect the night time supply to the live via the radio switch box. And theoretically could back feed the main house CUs via the live in connection to the radio switch. Is it safe to assume with a high degree of reliablity that the internal construction of the radio switch is such as to make this impossible?

(Obviously I wouldn't work on anything without checking it is not live, but the radio switch is, after all, a switch and if it could connect live to the wrong wiring could presumably do it suddenly and unexpectedly.)

[ARW]

With the main fuse out everything should be dead.

[Brian Gaff]

When they were working on mine they switched the house supply off. ahem. Careful prudent, safe? Brian

[Roger Hayter]

True, but I only want to change an MCB for an RCBO in the economy 7 CU and would prefer to rely on the main switch if it isafe to do so. Removing the main fuse is potentially hazardous, isn't it?

[Roger Hayter]

Where was the switch they used? Or did they take out the main fuse?

[Roger Hayter]

In response to a very sensible email suggestion, here are some photos. As an added bonus, does anyone think a Wylex RCBO will fit in the one unit CU photoed?

Thanks all for advice.

[ARW]

You cannot fit a RCBO into that CU. That looks like an old plug in fuse that has been swapped for a MCB.

Thanks for the pictures.

[Mike Humphrey]

OK, so it sounds to me like the 25mm wires carry the power. The 10mm ones power the timeswitch, and the 6mm one signals the meter to change rate. If you can find a model number of the timeswitch you may be able to Google somthing that will confirm it.

The supply that powers the timeswitch should be isolated from the switched terminals. If power flowed between them it would be dangerous for a number of reasons - as you say, the switch would not reliably cut power, but also with the switch open the electric heaters would run through a 10mm tail protected only by a 100A fuse. So I think you can be fairly confident that the switch does in fact cut all output power - only the timeswitch and meter stay live.

The consumer units should also have shrouds on the incoming terminals, so as long as the main switch in the CU is also off the CU won't become live even if power was supplied unexpectedly. If I was working on the input side of the CU, I'd be tempted to remove the main fuse anyway (though strictly this is illegal).

It's something to be careful of in multi-rate installations that there's often not a single switch to cut all power to the building. My house used to have THTC, which meant there were *four* main switches (general supply, 24 hour heating, water heating, timed heating). Unless all four were off, something was still powered or could become powered under timeswitch control.

Mike

[Roger Hayter]

OK so its a new CU or delete the economy 7 cirtuit. Thanks, that's useful to know. It is mainly borderline OCD that makes me want to change it just because it is the only circuit in the house not protected by an RCD.

PS I suppose I can't use an RCBO on its own without a mains switch a) to make servicing the RCBO safer and b) because I won't find an RCBO with

16mm terminals. Is there a more fundamental reason?

(I realise me doing it is all theoretical because of part P, of course, but I can influence the specs! Another practical electrician question is whether someone who replaced this CU would have to test the whole house installation?)

I've carefully avoided checking that the economy 7 circuit is wired as a ring so far, but I suppose I could always use a smaller MCB.

[Roger Hayter]

Thanks for confirming my thinking, I am much happier for a second opinion.

It looks as though the whole CU will need replacing, the one with the single, uncovered, MCB in the picture

That's complicated, did you have four separate incoming cables and main fuses, or was it all done after the meter?

[Mike Clarke]

Aren't E7 storage heater circuits radials?

[Roger Hayter]

The circuit only supplies two sockets, AFAICS, one for a tumble dryer (unused) and one for a single small storage heater. It is wired with

2.5mm T&E, so if it isn't a ring it probably wants a smaller MCB than the 32A one fitted. But I will get round to it, I've only been here 15 years.

[ARW]

Normally, although it's not a requirement.

[ARW]

A couple of options.

First of all find out if is a ring or not. Now is the time to find out!

If not a ring then swap the Wylex box for a new CU with 2 x 20A radial circuits with RCBOs or a RCD main switch. Don't consider a 2 way garage CU as you have 25mmm tails. You might just squeeze a 4way with main switch CU in there.

If it is a ring then

a) swap the sockets for sockets with RCD built into them. Job done in minutes.

b) swap the Wylex unit for a 32A double width RCBO in a suitable enclosure (going to be tricky with 25mm tails)

or

c) follow the advice for the radial circuit and have 3 spare ways and one 32A RCBO.

[Roger Hayter]

Thanks for the advice!

I've looked, and now wish I hadn't! The MCB is connected to a 10mm cable (could just be 6mm, but I think it is 10). However, by the time it gets to the sockets they are each wired with single 2.5mm cable. There is presumably some sort of junction box somewhere, but what he used that takes one 10mm and two 2.5mm wire I don't know. You'd think it would be big enough to spot! I presume branched radials are frowned upon, even with a suitable MCB. Fortunately I can scrap one of the sockets, as it is unused, being intended for running a washing machine/tumble dryer on Economy 7.

Actually, I don't benefit significantly from Economy 7[1] so I may just scrap the timeswitch[2] and connect the single storage heater to a spare way on the main CU, with a timeswitch at the socket. I expect they will charge me to change the meter.

[1] with my current supplier who I was allocated to when Iresa went bust I think I am paying significantly more on Economy 7. More round tuits needed. [2] very few supplers will charge one on a flat rate tariff if one has an Economy 7 meter, despite the simplicity of the arithmetic needed it hasn't been programmed into most of their billing systems.

[ARW]

I thought that suppliers were falling all over each other in a rush to fit meters for free?

It reminds me of a rewire I did some years ago when the customer had gas CH fitted at the same time as the rewire. Some months later she called back to say she was having problems with the electricity company and could I tell her the date on which we disconnected the economy 7 meter.

I could give her the exact date. September the 11th 2001.

[Roger Hayter]

No network signal.

[ARW]

The branched radials are in your case called unfused spurs.

They are frowned upon by some but not by others (including me).

If it worries you then without a doubt the easiest and cheapest option is to swap the MCB for a 20A MCB (you have to swap the back plate), fit a RCD socket and blank off the other socket into a flex outlet plate or into strip connector with a blank plate over it.

I am sure from your photos that this is the MCB you may need

BTW If you do swap the MCB to a 20A the unfused spurs become branches:-)

[Roger Hayter]

About 20m of 2.5mm T&E isn't a problem from the fault current POV I suppose? I could work it out but prefer the next solution.

Are branches acceptable in the context, with the common cable being bigger? I don't want one so that is an academic question. Thanks for the MCB hint, I think I'll get that pending a permanent solution.

[Mike Humphrey]

There's one incoming cable with a main fuse. This then fed a pair of henley blocks, which split the supply to two meters. One was just a single-rate mechanical meter, which did the "general supply" and fed a consumer unit for everything that didn't involve heating. The other meter included a radio timeswitch, with one continous output and two switched outputs. Although this was actually a multi-rate meter it only recorded one rate - all heating was charged at the same price. The three live and one neutral tails from this meter fed a second consumer unit with three independent main switches (each feeding their own bank of MCBs) that powered all the heaters.

The two switched outputs turned on and off according to a schedule that SSE set - it changed regularly, potentially being different every day, and customers weren't told what the schedule was. So unlike Economy 7, which is unlikely to turn on at a random point in the day, it's extra important to make sure that the relevant main switch is off - as you never know when the timeswitch will switch on! Just to add to the confusion, the immersion heater and some of the storage heaters had a boost facility - fed from a different circuit on a different main switch.

Mike