Replacing Consumer Unit

[lloydwatkins]

In preparation for an extension to my house, I wish to replace my consumer unit (wired fuse type) with a larger one with spare ways for the circuits in the extension. The CU replacement will be fairly straightforward but I have a couple of items I wish to check first.

The power reaches the house by way of a long overhead cable, running underground from the very last post to our house. Incoming is a fat black cable which splits to two smaller but still fat cables running into the company fuse and a henley(?) block.

From these two blocks I have fat red and black cables to the meter,

then same large red & black to a Chilton Current Operated Earth Leakage Current Detector. Finally cables run from the ELCD to the Consumer unit. A separate earth cable runs from the CU to an earth spike. (I will replace this with a much fatter earth cable at the same time as it looks a little thin to me)

I wish to have a split load CU, with the usual circuits on the RCD side and lighting/freezer on the other side. Is this ok ? and do I have to leave the ELCD in the circuit or can I connnect the new CU direct to the meter ?

Finally I want to run separate RCD circuits to the shed (back garden) and garage (front garden) from the spare ways in the CU. Should these be on the RCD side or the other ?

LHW

[Andrew Gabriel]

This is a TT installation. Current Operated Earth Leakage Detector is the original name for an RCD -- does it actually say that on it, as Chilton are better known (to me at least) as a manufacturer of Voltage Operated Earth Leakage Detectors, which is a previous generation device. What rating is written on it? The earth wiring you describe is however consistent with an RCD.

As you have a TT installation, all your circuits will need to be RCD protected. It would be worth asking your supplier if you can be easily converted to TN-C-S in where this would not be the case, but that depends if the local supply network has been converted to to Protective Multiple Earthing.

If you remain TT (own earth rod), you can still use a split CU, but both sides will need RCD protection. One is protected at 30mA for the circuits where protection against electrocution is required or desirable, and the other is protected at 100mA (and can be time- delayed) where protection against electrocution is not required (it has to be time-delayed if the 30mA side is daisy-chained off it). If you have the space available, I would consider doing these as 2 separate CU's, which gives you slightly more flexibility.

You generally don't want circuits running outdoors sharing an RCD with indoor circuits, as outdoor faults are more common (moisture ingress), and could leave indoor circuits unusable. Socket outlets in your shed and garage should be RCD protected as they're likely to be used to power outdoor appliances. This really implies they should be on the 100mA RCD side and protected by an additional 30mA RCD. If you were going the separate CU route, then add a third dedicated 30mA RCD protected CU for them. There are lots of options here, depending what you will be running in there (a freezer, for example?) and how dry they are. If you do end up sharing an RCD between indoor and outdoor circuits, do at least include a 2-pole isolating switch in the outdoor feeds so you can isolate a faulty circuit.

[lloydwatkins]

Thanks Andrew, just the type of clear advice I was hoping for.

Having read all the other threads about ELCD I was very careful in reading the data plate on it. It clearly reads Current Operated and is a Type E (could be Type L as the typeface is not the greatest). It is rated at 500mA if that means anything.

I will ask the supplier about the incoming supply and see how easy (costly) the conversion would be. I quite like the 2 x consumer unit options, I have the space do I just run two pairs of cables from the meter, or do I daisy chain it someway ?

For the 'outside' circuits, assuming I go the separate 100mA CU route, can I dedicate a way(or 2)in the CU for each outside circuit and put

30mA RCDs directly in the CU (or perhaps RCBOs) and run a ring from that or do I have to run from a MCB to a 'garage' CU with its own RCD ?

Lloyd

[Owain]

If you do this as part of the extension project and include it on the building regulations application, you will be covered for the purposes of compliance with Part P.

Owain

[Andrew Gabriel]

OK, so it's a 500mA RCD. Presumably, it has zero or just one earth connection point. A Voltage Operated ELCB has two separate earth connections, one to the earth rod, and the other to the CU or earth terminal.

You use a service connection block (Henley Block) to make the connections of the tails. You would probably not get multiple tails into the meter connections. Also, you probably won't be able to access those connections without breaking the meter seal, which is a big no.

You could do either.

[Les Desser]

In article , [e-mail] Sun, 4 Mar 2007 02:45:47 writes

I can never understand the justification of using a single RCD for all/most of the circuits in a house - other than some saving in costs.

Who in their right mind wants a whole house to go dark just because of a fault in one area?

I recently was called over to a neighbour where everything had gone off as the ELCB had detected a fault. In the end it turned out to be the central heating circuit - probably a faulty immersion heater.

I know that a standard CB is about £5 and a combined CB/ELCB is about £20 but I would always advocate the use of the latter. For an extra £100 or so (+ VAT) each circuit could be individually protected for earth leakage as well as over-current. Considering the total costs involved in rewiring and the extra cost of the above, I cannot understand the budget solution even being allowed by the building regs.

If you can, put the fridge and freezer on separate independent circuits for maximum resilience. If a kitchen appliance develops a fault, your fridge and freezer will continue working.

[Andrew Gabriel]

I agree. In the last 3 houses where I've replaced a CU, I've used a non-RCD CU, with several RCBOs on the circuits where I want RCD protection. It's not cheap, but that's what I spend the money on which I saved by doing it myself.

None of my CU's have RCBO's available that cheaply. £25 - £60 is the range I've paid (top end was rather specialised).

And central heating and any other frost protection devices.

I usually have two circuits in a kitchen. Non-RCD protected for stationary and fixed appliances which don't represent a significant electrocution risk, and RCD-protected for portable appliances and any other easily accessible socket outlets, which is where the electrocutionm risk is significant.

[Lurch]

On 4 Mar 2007 02:45:47 -0800, [e-mail] mused:

Fat black cables? Fat red cables?

Leave it alone, you're scaring me. Time for the Yellow Pages, much safer.

[John Rumm]

Indeed, ("whole house" RCDs) a practice now depreciated

Typically £40 or more alas. Another issue is that many are two modules wide (or extra tall and only fit some CUs)

It isn't permitted in the wiring regs - hence why split load CUs are now the norm.

Good plan to stick the CH boiler on one as well - save anything freezing up should you be away on hols. (oh, and tropical fish tanks!)

[John Rumm]

Don't take this the wrong way, but in common with some of the other posters, I get the impression from some of your descriptions that you might want to brush up on some of the finer details before taking on this job, as there are a number of subtleties that can trap the unprepared!

Might I suggest a copy of the IEE On-Site guide as a basic starting point? Amazon link:

I wish to have a split load CU, with the usual circuits on the RCD

As Andy suggested two CUs is often a simpler way to do a TT system -

100mA trip RCD in one and 30mA trip in the other. It is often better to split out outside circuits yet again to save the risk of having them trip the house circuits. If you are having multiple CUs then a single master switch positioned before a henley block is a good way to fan out the meter tails to each CU. That way you still have an overall "off" switch should it be needed in a hurry.

My solution in your situation was to have two CUs for the house, and then a another dedicated time delayed one for outbuildings (which in turn have their own RCDs). The outbuildings do not have split load CUs, but instead have non maintained emergency lighting where appropriate.

[robgraham]

Could I also add in that if the run to the external CU's is lengthy, then it is desirable to put in another earth rod at that CU. Rods in parallel will improve the system earthing.

Also before you go upgrading the earth wire, bear in mind that the earth resistance is quite high and the voltage drop across it will far exceed that across the earth lead, unless excessively long. If you search on this NG you will find a method of measuring the earth resistance - unfortunately I stored the procedure as a .txt file and can't point you to it. Post back if you can't find it and I will paste it up.

Rob

[Les Desser]

In article , John Rumm Mon, 5 Mar 2007 03:11:59 writes

Sorry my memory at fault. £33+VAT at Screwfix for MK

[Andrew Gabriel]

I've got those for £25+VAT each by buying a box of them from a wholesaler (5 in a box, IIRC).

[lloydwatkins]

Rob, I don't quite understand your point about upgrading the earth wire. I had thought that a larger earth wire and possibly an extra earth rod would always be a good thing, are you saying that a larger wire with (presumably) lower resistance would increase tripping of the RCD or is there a technical problem that I don't understand ?

LHW

[John Rumm]

I think he is saying that the size of wire to the main earth electrode is not actually that important since it only has a small part to play in the grand scheme of things.

Say your earth electrode (rod) has a fault loop impedance of 10 ohms (which is not bad as these things go). A 6mm^2 wire (the size specified in the in-site guide for TT systems), would introduce a further 6 milli Ohms/meter extra. So to all intents and purposes the size of the earthing conductor makes no significant difference to the total earth fault loop impedance.

Adding additional earth rods *may* help reduce the total earth loop impedance (if far enough apart), but again that may not be much use unless you have a very high (or seasonally variable) earth loop impedance to start with. In the case of an outbuilding it can be worthwhile making the outbuilding a different TT system from the house - i.e. its own earth rod etc. You would then need to ensure the CPC of the submain to the outbuilding is not connected at the destination end.