Reconfiguration of a (TT) Consumer unit to Split Load

[Peter]

I think this is my first biggish question to the group! Having read similar threads so far I don't think the following has really been covered.

Introduction

------------ At my mum's house we currently have a Off Peak supply which was being used for water heating, washing machine, dish washer and tumble dryer. The new heating system will be taking over the hot water duties. Thus we are planning to discontinue the off peak tariff.

We have a TT installation with our own earth rod and over head supply.

Current

------- The current consumer unit is a surface mounted 16 way insulated MK sentry "s" series (the range before the current with the smoked gray transparent lid). It's configured as a multi-incomer with the "mains" and off peak supplies as below

way(s) 1 - 2 80A 30ms RCD "Mains" incomer way(s) 3 32a MCB Cooker way(s) 4 16a MCB Central Heating way(s) 5 32a MCB Ring Main 1 way(s) 6 32a MCB Ring Main 2 way(s) 7 16a MCB Outside Sockets way(s) 8 6a MCB Lighting 1 way(s) 9 6a MCB Lighting 2 way(s) 10 blank way(s) 11 blank way(s) 12 - 13 80A 30ms RCD "Mains" incomer way(s) 14 32a MCB Washing Machine and Dishwasher radials way(s) 15 16a MCB Tumble Dryer Radial way(s) 16 16a MCB Immersion Heater Radial

(you now know what components we've got).

Proposed

--------- As I mentioned we would like to remove the off peak tarrif and change the system into a split load configuration. So far I've come up with the following configuration (which will involve a bit of re-wiring in the house).

way(s) 1 - 2 100A Switch "Mains" incomer way(s) 3 - 4 80A 30ms RCD Split One way(s) 5 32a MCB Cooker radial way(s) 6 16a MCB Central Heating radial way(s) 7 16a MCB Fridge & Freezer radials way(s) 8 6a MCB Lighting 1 radial way(s) 9 6a MCB Lighting 2 radial way(s) 10 - 11 80A 30ms RCD Split Two way(s) 12 32a MCB Ring Circuit 1 way(s) 13 32a MCB Ring Circuit 2 way(s) 14 32a MCB Ring Circuit 3 way(s) 15 16a MCB Outdoor Sockets radials way(s) 16 16a MCB Immersion heater radial

Comments

--------

- Each RCD will be fed via the one Switch disconnecter (live and neutral). This in turn will be fed from the Boards meter.

- Since we already have the 2 RCD's it's cheaper to buy a switch disconnecter than a time delay RCD and cascade the "split two" circuits as per conventional set ups.

- TT set ups require everything to be protected via a RCD

- The clear advantage is that all circuits are protected via a 80A 30ms RCD (as they are at the minute with no trouble trips yet - just a duff Iron), instead of a slower 100ms Time delay RCD.

- Where Ring Circuit 3 (could/would) be a new ring formed around the utility and adjoining garage. This would include the Washing Machine, Dishwasher, Tumble dryer sockets (see below).

Queries

-------

1) Does this sounds like a good plan or is there any concerns or suggestions for improvements?

2) The incoming Switch disconnecter will need to have a connection to the two downstream RCDs for the live and neutral connections. Is it permissible (or even possible) to connect two 16mm sq tails into the cage connectors on the switch (50mm sq capacity)?

- For the short distance between the Switch to the RCD could 10mm sq tails be used (pretty sure the answer is no here!)

- Some form of crimped furrels

- external henly blocks - ugly

- would getting an extra MK split load kit be advisable and use the extra cables (these have crimped ends and are much more flexible)

3) The above mentioned white goods (Washing machine, Dishwasher and Tumble drier) are currently connected with standard single 13a sockets and all wired back to the CU with 2.5 mm sq T+E. But into a 16a and 32a MCB. To save hassle could these remain as radial circuits back to a single 32a MCB (The total loading on these would be more than 32a, if all are on together ~8Kw max, relying on discrimination here)? Can 3 radial circuits, on 2.5 mm sq, be suppled via 32a MCB? In this case the remaining sockets in the utility room and garage would be left as part of the existing ring circuits.

Personally I do reckon it would be best to convert in to a new proper ring - just more effort on my part.

I've got over a week before I have to start crawling around roofspace and doing this work, and I reckon it shouldn't take more than a day or two.

Thanks in advance for assistance.

Peter.

[John Rumm]

Congratulations on a very complete and informative post!

You may find it simpler to stick the mains incomer in an external box before the CU.

I would make this a 100mA trip RCD rather than a 30. Since the collection of things you have here are the ones you least want to trip!

Looks OK...

Assuming you went for the higher trip current RCD on the other split, then the immersion heater could go on that side since these can be high leakage. You may also move any final ring circuits to the higher trip side if they are unlikely to be used for powering portable devices used outside.

I take it you mean *both* RCDs will be fed from one switch. (it is a requirement to have a single "main switch" somewhere that can turn of the whole system

Might also be worth checking that the earth loop impedance to your earth spike is reasonable (i.e. under 200 ohms).

Since you are not cascading, there is no need for a time delayed type to achieve discrimination. You may not however want such a low trip current on the less risky circuits, hence my suggestion for replacing one of your current RCDs with a higher trip current one (earth loop impedance permitting)

Can't see any obvious show stoppers...

Will probably depend on the switch... you could add a Henley block after the switch.

Does not sound like a good idea. ;-)

Not keen in this situation...

Would be my choice

Can't see any real advantage. You could get an additional CU and install both. One designated as high trip and the other as low. The only real advantage here would be having some spare ways for future expansion.

Not quite sure I understand what setup you have there... one MCB per appliance, or one on the 16 and two on the 32?

The word you are looking for might be "diversity"... You can only rely on it, if you are not aware of a likely possibility that the overload situation may occur.

The problem here is that given enough load the cable will die before the MCB trips. This is why a high load radial like this would typically be wired in 4 or 6mm sq cable.

I think that would be best... alternatively you could uprate the cables and/or down rate the MCBs to make the radials safe.

[Stephen Dawson]

All I will say to you is that you must on a TT installation protect all sockets outlet at 30mA, not as john has stated. I would however change the main switch and fit a 100mA Time delay for the rest of the installation. If the job is worth doing, do it properly. Don't forget to test your earth pin and rcd's either, as they may not work !!

Regards

Stephen Dawson

[Jim Warren]

If you want to go ahead, then the other replies in the thread give you the information to do it.

But - have you thought about keeping the off-peak tariff and done the arithmetic? A washing machine, even one with hot fill, usually fills the drum with water at a lower temperature than the final wash temperature and then heats it further. This is to allow time for the enzymes in the washing powder to do their job before being destroyed by the temperature. A dish washer is always cold fill and it heats it to a high temperature during the wash. A tumble dryer heats air to dry and vents it (or cools it with a condenser). Your new heating system will make very little difference to the amount of electricity used by these appliances.

So if you keep the off peak tariff and invest in a couple of time switches to start the appliances during off peak times, you could save yourself some money, well in excess of the premium you pay for the off-peak arrangement. I know somebody in your position, having moved into a house with off peak tariff arrangements and then upgrading the heating and hot water, and when they did the sums it was definitely better to keep the off-peak.

Jim

[Andrew Gabriel]

This isn't true. However, when you have more than one mains switch, you should fit a notice saying that ALL main switches must be switched off to isolate the installation.

No, 100mA is fine here, as the protection in this case is not against electrocution (which is where 30mA protection or lower is required).

Of course, some socket outlets may require 30mA protection for other reasons. Personally I protect all socket outlets at 30mA unless they are for a dedicated appliance, in which case I position them so as to be pretty inaccessable for any other purpose.

[Andy Wade]

All quite correct/agreed of course.

More points:

Before doing anything check with the supplier / distribution network operator whether PME is available. Many overhead supplies have now been upgraded to multiple earthing. If PME is available, use it, with a conventional non-RCD / 30 mA RCD split-load arrangement in the CU. (Install a temporary outboard RCD in the tails until 'the board' come along and transfer your earth to their PME terminal.)

[The rest assumes no PME option.]

The main switch plus two RCDs is my preferred way of doing this, and avoids the need for a time delayed RCD. For the current Sentry product generation, MK do a 'main switch & 2 x RCD cable kit' which sorts out the heavy-current side of the wiring. Whether this can be used/adapted for older CUs is debatable and I'd suggest a call to MK's technical help line, which I've always found to be helpful and knowledgeable. They can also sometimes supply older generation parts.

Otherwise, yes, you can do the split using 16 mm^2 cable, although MK will point out that this invalidates the BS type approval for 16 kA fault rating. Unless the supply fault level is very high - highly unlikely on a rural overhead supply - I wouldn't worry too much about this.

IMO the only circuits that should be on the 30mA RCD side are those feeding socket-outlets likely to be used to feed portable equipment outdoors [see OSG section 3.6.2 and Fig 3a]. Everything else should go on the 100 mA side. A separate 100 mA RCD (or RCBOs) for the lighting circuits is even better - but you then need to work out how to do a

3-way split after the main switch!

This doesn't apply here since this CU is plastic, but remember that with a metal CU housing in a TT installation you must meet Class 2 insulation requirements between all pre-RCD wiring and the housing - a phase-earth fault before an RCD would make all the earthing live and heat up the earth rod, and there's nothing to clear the fault. (See MK RCD insulation kit.)

[John Rumm]

Fairy snuff, seems I read too much into the requirement to label the "main switch" as such... ;-)

[Peter]

It's going to come down to a matter of convienace more than cost, and I've just had a chat with my mum about this and then did some sums.

The washing machine is on cold fill only. We reckoned it would take to long for the hot water to come through the house to the washing m/c to make it worth using the hot fill (my new Siemens washing machine doesn't have a hot inlet).

But we are on an old preserved tariff from NIE which is quite expensive. Unfortunately the off peak devices are only supplied between the off peak hours (which are quite generous) so they can not (should not) be used during the day (no contactors or plug sockets allowed I think). Also the Bosch washing machine loses it's program settings if the power supply is interrupted. So mum tends to plug then into a nearby normal socket instead!!

I we are on a NIE 15 hour tarrif, which has a ~£8 standing charge/quarter and is billed at 5.64p/unit. There is no standing charge for there "home Energy" tarrif, which is billed at 9.64p/unit. Thus the break even point is at about 200 off peak units/quarter. (

)

For the last two quarters (were the hot water was heated via the electric immersion) between 535 and 565 off peak units were consumed.

By my very rough reckoning heating/maintaining a cylinder of hot water takes about 5 units per day so over a ~90 quarter that's 450 units. Leaving about 100 units used for the 3 white goods.

Mum lives on her own now so i guess that the white good would be used more with a full house (although she just told me that my sisters dumped a load of washing around!).

Jim, thanks for making me do the sums.

Peter.

[Peter]

I've just phoned up the REC and they are going to send out a "Provision of earth" form which needs to be filled in before they can get a planner to see if it is possible.

I've just check out the website and the have a kit for the current k series units K5564s, which is as you described. I must have missed that before. Anyway there tech support line does not see any problem with using it on the previous series of CU.

Fantastic, _if_ I can get hold of it, it'll make things a lot easier. (any good MK Sentry retailers in the Reading or Belfast Areas?).

If I was to have a number of rcbo's after the main incomer it would be a lot easier to supply the two RCD by using MK's 5562s extension terminal. Taking a feed to each rcd from different points along the busbar. But I would need a third neutral block. MK used to have them in there parts list.

Noted, that's the kit which looks like a two slot clamp.

As a summation of the replys so far I am going to:-

- pursue the PME options / Check current earthing arrangements [1]

- Cease the Off Peak Supply (see my previous reply)

- use two RCD's down stream of the Switch disconnecter and depending on availability of the MK kit or the confidence I get with a pair of 16 mm sq tails out of the Switch dis will determine if the switch sits in the CU or in a separate unit with Henley blocks.

- If the 30ms trip current of the RCD does give some neusance trips i can swap it out for a 100ma trip for the not portable appliance sockets circuits and re-gig the CU. But from experience 30ma trips have been ok so far.

- install a dedicated radial for the fridge and freezer from one 16a mcb

- establish a new ring circuit for the utility room (containing the white goods) and the garage (no real loads in there). Maintaining radial circuits for this is going to be more of a boge. Bit concerned with having the white goods on one 32a ring, but with diversity[2] and MCB trip curves it should be fine.

- new labels for the CU

Anyway, Thank you all very much for your help and assistance

Peter.

[1] How is the earth loop impedance tested, resistance between earth rod and the supply neutral or a plug with suitable resistance between live and earth pins to draw ~30ma? I tried googleing it in this newsgroup but didn't find too much detail. However the system was checked out by an electrician a few years ago. [2] Yep diversity is the word I meant, you would hardly believe that i worked at norweb for a year during university in there distribution protection dept (11kv downwards sorting out descrimiation between cascaded levels of circuit protection equipment (11kv circuit breakers, 11kv reclosers, 11kv fuses, transformers, 440v fuses etc...).

[John Rumm]

One thing to remember about RCBOs is that many of them are two modules wide, hence you can run out of ways in the CU rather quickly. (they are also a tad pricey if you need many of them!)

Not trying to sound sarcastic, but, with an earth loop impedance tester! ;-) Megger and the ilk do said beasties, Typically they plug into a 13A socket, and have a test button. You push it, it tells you the loop impeadance. (I have one you are welcome to borrow, should you be in range of SE Essex).

(You can't use a DMM because the test needs to be done at mains voltage, also the testers are designed to do the test fast enough to not trip the RCD)

Easy enough to do... I can go for days not being able to remeber the exact phrase I was looking for ;-)

Makes 240V all seem a bit "tame" I guess!

[Andy Wade]

Boggle - "provision of earth" indeed. The Magratheans provided the earth; all the REC have to provide is a connection :-)

Any electrical wholesaler should be able to order it for you, if you can't find one with it in stock. Newey and Eyre branches always used to have good stock of bits 'n pieces like that.

Yes the separate neutral block that mounted on the DIN rail seems to have been discontinued. The single module RCBOs have flying lead incoming N connections - so it's down to how you can make a sound connection to those. They might work in the screw terminals of the main switch and/or RCDs - but combining large and small conductors sizes in one terminal is always potentially problematic. Don't risk any loose connections, obviously.

Plus a hefty bushing for where the incoming tails enter the box.

If you're talking about measuring the earth electrode resistance (which of course is the dominant part of the loop impedance in a TT system) then

give you some ideas.

[Stephen Dawson]

I stand corrected, have just looked in the book(OSG page 21, Brown Cover), and 30mA RCD should be used to protect ccts to portable equip outdoors , sockets outlets that may reasonably supply portable equip outdoors (ground floor sockets).

[Jim Warren]

And I can see why you are going ahead. The person I know is on Economy 7 so their arithmetic is rather different.

Good luck with the rewiring.

Jim

[Peter]

heh, they must feel very self important with forms like that. :-)

I went around to the local Newey & Eyre in Reading and picked up the "2 rcd kit from Switch Disconnecter" kit for £5:60 ish.

I remember going to elecy wholsalers in the past and asking for something and they look blankly at you or say "We've stoped selling MK Sentry now".

It took a trip around a few wholwsalers to get one of them a number of years ago.

From reading around the subject it seems like this isn't going to be much of a problem as it's not difficult to get a sufficent earth connection with a rod. The rod in question is stuck in damp ground around the back of the garage. I stuck it in years ago, but we've still got bonding to the incomming water main!

I'm gonna see about borrowing a tester anyway.

Thanks again for the advice.

Peter.

[Peter]

Thanks for the offer, unfortunately the installation is near Belfast (I live in Reading and I'm flying back for a bit and this is one of my tasks to get sorted out whilst I'm back). My new Brother - in - law probably knows an electrician so I might get the loan this way.

The best bit of that was when visiting the Grid/Bulk Supply point in Manchester next to their office. We were being shown around and before we went into the 33kV 'breaker room' we were given all the warnings about don't touch anything, there are live busbars, and if a breaker were to trip it would be very loud etc... We were then politely let in in front of our guide. When out eyes got accustomed to the low lighting we realised that the busbars were inches above out heads! All insulated of course, but still gave us a a chill down our spines.

These 33Kv oil or gas filled breakers are huge, about (from memory) 9 foot high 2½ foot wide and 1 foot deep. With sections for (three phase) busbar in/out, circuit out, CT coils and of course the breaker. Up stairs was the (kinda abandoned - due to automation) control room. In there were a set of very ordinary looking 3 phase meters - even the RECs get elec bills These of course were fed from the CT. Beside these were the various trip relays (IDMT etc...).

That was a very good project to work on.