Is an RCBO an adequate replacement for an isolator?

I'm looking at replacing a somewhat old consumer unit. I'll freely admit that my electrical skills are not up to it, and I probably don't have all the necessary test kit, both of which are a shame.

Anyway, a local firm has scented an opportunity (all the places on the same development have the same, old, consumer unit) and have sent round a fuzzy before and after picture of what replacing with a new RCBO based unit would look like, and estimated a standard price, with an addition if you need more 'ways'.

Anyway, it looks, from the picture, as though they'd remove isolating switches and seem to have a design that has:

a) an RCBO per way; and b) an RCBO acting as a 'main fuse', sized to be 'equivalent to' the current main fuse.

Hence the question in my title. Is an RCBO a suitable isolator? And is the rest of the design sensible? I can't tell from the text, or the fuzzy picture, but I would assume that the RCBOs per way would be fast acting, and the 'main fuse' replacement would be slow acting - otherwise a nuisance trip could take out the whole board, which is what I though we were trying to move away from.

If the supply is rated at 60A, is that what they would propose for the 'main fuse' equivalent? My understanding is that RCBOs (and MCBs) are less 'forgiving' of overcurrents than fuses, so I'd actually lose some peak capacity (not that I use it, but some might).

Of course, I can put the same questions to the firm that put the flyer through the letterboxes, but I'd prefer some independent opinion first, if that's possible. Thanks!

Sid

Reply to
Sidney Endon-Lee
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Are you sure about the "main fuse" being an RCBO? A double pole RCD would be more the expected item and to provide discrimination would need to be time delayed. Otherwise any significant earth fault on any circuit would trip that cicuits 30mA RCBO "and" the main RCD cutting power to the whole house. The use of a 100mA typa S, time delayed RCD as main switch or isolator is perfectly acceptable. It does beg the question why you think you need to replace your original consumer unit apart from cosmetic reasons do you have any genuine concerns about your installation? I'll bet the firm will also be hoping to pick up on all sorts of extra works based on things they "find" once they get started.

Reply to
cynic

17th edition CUs have a conventional 2 pole main isolator switch and two RCDs feeding a busbar each. On those busbars you have a choice of using MCBs or RCBOs.

As a general point I'd be wary of using a firm just on the basis of a flier. Unless neighbours have already used them and can vouch for their work.

Reply to
Dave Plowman (News)

Sidney Endon-Lee wibbled on Wednesday 11 November 2009 10:10

Which provides the same level of isolation as the MCB that might be there in a different, but complaint, installation.

Your "main fuse" is the one in the elect co's cutout, typically near and definately before the meter.

A time delayed (type S) RCD may be used in place of the main CU isolator, or before it in a separate box, if you have a TT (earth rod) installation earth. This is not and RCBO - it doesn't deal with current overloads and it doesn't need to, any more than the main 100A switch in a CU would. The supplier's fuse does that.

You could also have a non delatey 100A RCD - but this would be essentially stupid along with RCBOs.

As much as any equivalent device that might be used is. It will typically only isolate L and not N (although I have two double pole RCBOs that do isolate L+N for outside circuits, because it seemed like a good idea, though beyond basic requirements).

This means that you should not use the RCBO to act as eg as a shower isolator - you still need the local DP switch. But in every other respect, the RCBO is a complete replacement for and MCB or fuse.

Yes - 30mA earth leakage trip in less than 40mS IIRC - same as any normal domestic RCD. The current overload curves will be as fast or faster than a typical domestic fuse and typically the same as an MCB (note there are different curves, B, C and D: B is typical in domestic and C is occasionally found,being a bit "slower").

Yes - in respect of the RCD trip time. The local per circuit RCBOs should trip first and not this unit. The only way this should happen is if the combined earth leakage is high enough but the contributing components are too low to trip the local RCBO; or if the fault is between the RCD and RCBOs.

Exactly. But do you need the main RCD? Is the place TT earthed - or are they proposing a split board with the RCD protecting a few random circuits and the RCBOs bypassing the RCD? In which case the main RCD should also be

30mA/40mS.

Not relevant as the main RCD doesn't do overcurrent protection.

I'd be wary of responding to a random flyer. If you want the work done, you might be better getting another quote or two. OTOH, the company *may* be genuine and you *may* benefit from them doing a bunch of identical work in the same area (which is obviously of benefit to them too).

HTH

Tim

Reply to
Tim W

Remove which isolating switches?

Fair enough...

This bit does not sounds quite "right"...

Assuming you have :

1) A main cutout (traditionally a 60A or 100A fuse on the suppliers side of the meter), tails from these feed the meter.

2) Tails from the meter then either feed the consumer unit directly, or possibly via an isolating switch (and on complex or augmented installations, there may be other CUs fed from tails split in a junction box)

3) A CU which would normally have a main switch of its own (or perhaps a main RCD acting as a switch), followed by a number of circuit breakers or fuses.

I would expect a RCBO based replacement would be the same up as far as the CU, and then the CU would have an ordinary main switch, followed by RCBOs for each circuit.

Its not clear from your description if the "local development" is a collection of independent houses etc or a multi occupancy building. With the latter the "suppliers" side of the meter may well belong to the freeholder rather than the supplier, and hence replacing the main cutout with a MCB would be more plausible.

Generally no, since most modern (slim) RCBOs are single pole devices, and hence don't isolate in the true sense. You can get double pole RCBOs (usually double width modules). In the occasions where you would want RCD functionality in the main switch location, you would probably not want the overcurrent protection of a RCBO. Also you don't normally cascade RCBOs (especially since time delayed RCBOs are somewhat rare)

MCB etc are actually very forgiving of over currents, but tend to be faster on fault currents. (loading a 32A circuit to 40A for 20 mins is an over current, nailing through a cable creating a short circuit passing 800A is a fault current). So I would not see that as a particular down side as long as the device is sized suitably to discriminate with any of its downstream circuits.

Reply to
John Rumm

An RCBO can eliminate equipotential bonding in bathrooms. I believe the lights and power all have to be off the same RCBO.

Reply to
Doctor Drivel

Why would you want a main RCD if you have double pole RCBOs on all circuits? I can't see the need for an RCD in this situation.

Reply to
Doctor Drivel

Than-you for all of the very quick replies. I hope I can clear a couple of things up

My place is one of a large development of purpose-built flats, built by the GLC in the 1970s. They all have Wylex rewireable fuses in the consumer unit, which has a 60A incomer. When I moved in, there was still an original GLC card of fuse-wire in the cupboard with the consumer unit. I've not had to use any of it myself, other than for an electrician to add another 'way' to correct a d-i-y bodge I found in the kitchen (unfused spur off a socket with three sockets off the single ring, said spur providing power for the dishwasher, washer/ drier, and sink waste-disposal unit, with a Fridge/freezer plugged into the ring socket.)

As far as I know, the supply is not TT. Whether it is TN-S, TN-C, or TN-C-S, I have no idea.

As for why replace it, I like the idea of incorporating RCD protection, so I'd been thinking off and on about doing it for quite some time - the flier just crystallised some of my thoughts.

The flier has a fuzzy picture, so I may have mistaken a simple 'main switch' for an RCBO - the only reason I know they are RCBOs is because that's what they say in the text, rather than MCBs. My preference would be to have individual RCBOs if I am replacing what's there, as I run a fair amount of IT equipment/switch mode power supplies, and I'd like to have enough headroom for leakage currents.

I've learned something about MCBs (thanks John) and overcurrent. Some people I've spoken to have preferred fuses because (summarising their words) "they tolerate a fair degree of overcurrent, and don't weld the contacts together in fault currents"

Thank-you again all.

Sid

Reply to
Sidney Endon-Lee

I would tend to go for double-pole RCBOs on all circuits. And, I'm this is right, have the bathroom's light and power off just one and I know it is easier said than done, that is why some have an RCBO on a surface patress over the bathroom door in the hall. I think double-pole RCBOs is standard in new installations in Germany - they use radial rather than ring circuits. More expensive but worth it.

Reply to
Doctor Drivel

I'll just make a small observation, which I know is not comparing like with like, but it seems quite amazing to have gone from fuses with a replaceable element that (in the case of rewireable) costs a few pennies, to dual-pole RCBOs which cost 70-80 pounds a piece!

I understand the some of the safety advantages - can't replace the fuse with a hairgrip, RCD is good. And there's the cost of fitting things into DIN format. It's still quite a contrast.

Sid

Reply to
Sidney Endon-Lee

Good way of ending with a mile-long CU & higher cost.

Standard in Italy & Germany. Specifically because polarity is not indicated nor guaranteed.

Nothing to do with this, although several radial offer better compliance with 314.1 than a single ring. A ring final circuit however offers some safety benefit over radial re 2 paths to earth. A radial final circuit of 20A offers some safety benefit over ring in that a double 13A socket can actually only supply 19.5A yet has a 32A CPD. Not a problem except in kitchens with few sockets and lots of heavy appliances where Things Can Get Toasty.

Not worth it at all. Fixed appliances have their own DP isolator, then lockoff the relevant MCB/RCBO or main switch).

- Have an incomer Main Switch (DP Isolator).

- Have RCBO per circuit or 3x RCD (makes for a long 21-way CU which requires a tube trip to get from one end to the other).

- Ensure at least 2 spare ways.

Also what brand are they - I ask in case one fails and you can't get replacements re maintaining type-approval of existing board. Contactum, Hager, MK, MEM, MG, Square-D are all a "known quantity" whereas there is some real grotty stuff out there.

Make sure electrical tests (RCD in particular) are done on *your house/ flat* vs done on one and copied for the rest as can happen because it can save a lot of time and a QA spark is busy looking the other way / too busy / not checking.

I assume you have a smoke alarm? Reason I ask is depending on the cost things might be better "spread around".

I assume your wiring is modern PVC, not Pyro/MICC or TRS rubber? If the latter you may get involved in a rewire of partial circuits, one reason they might be suggesting RCBO so they can get "some circuits back on" and fight any remedials with the remaining. Pyro is fine, although it has some short comings (if people DIY'd in the past they might have stuffed something enough to make an RCD trip).

Reply to
js.b1

Sidney Endon-Lee wibbled on Wednesday 11 November 2009 14:32

Whilst a rewireable fuse isn't inherently bad *providing* no-one lashes in the wrong size wire or a nail (which is the main criticism, along with the lack of ease of resetting them) - but the lack of RCD protection is well worth the effort of upgrading the CU. It's also an excellent opportunity to assess the state of the wiring via the PIR type checks that anyone changing a CU should be doing. 1970's wiring should be fine, but checks could still uncover problems or even original errors like broken rings and dodgey earths.

However, this might be something to watch out for - is the company actually on the lookout for additional work, with the risk that they might invent problems that can only be cured with a full rewire. Worth being ready to challenge them on that, should it happen. Make sure they don't start any work wihtout an agreed price and scope.

Then there is no perceiveable reason to have a main RCD. They might be proposing a split board with one main switch, feeding a bunch of RCBOs down one side for sockets and then an RCD protecting a bunch of MCBs for lighting or some variation on that scheme.

Reply to
Tim W

Doctor Drivel wibbled on Wednesday 11 November 2009 14:56

Yes they are considerably more expensive (and harder to source), and usually take up 2 ways rather than 1 in the CU. If they were cheaper and slimmer, I'd agree and have done it myself. As it was, I could only justify DP on external circuits as being a decent compromise.

Cheers

Reply to
Tim W

Yes, in fact more than one.

All the wiring I have seen is PVC. A previous house had unshuttered BS

546 sockets with exposed conductors, rubber covered-wiring, and also lead-sheathed paper-wrap. The fuseboxes under the stairs were a wonder to behold. The landlord didn't care.

The electrician who rectified the kitchen bodge ripped out and made safe the other bodges we could find, including the classic use of bellwire to carry power to a cupboard light.

Regards,

Sid

Reply to
Sidney Endon-Lee

Good way of ending with a mile-long CU & higher cost.

Reply to
Doctor Drivel

Nothing to do with this, although several radial offer better compliance with 314.1 than a single ring. A ring final circuit however offers some safety benefit over radial re 2 paths to earth. A radial final circuit of 20A offers some safety benefit over ring in that a double 13A socket can actually only supply 19.5A yet has a 32A CPD. Not a problem except in kitchens with few sockets and lots of heavy appliances where Things Can Get Toasty.

Reply to
Doctor Drivel

As long as all electrical circuits that enter a bathroom are RCD protected then the supplementary bonding in the bathroom may be ommited. There is no need for all the circuits to be supplied by the same RCD or RCBO though.

Adam

Reply to
ARWadsworth

Good way of ending with a mile-long CU & higher cost.

Standard in Italy & Germany. Specifically because polarity is not indicated nor guaranteed.

Nothing to do with this, although several radial offer better compliance with 314.1 than a single ring. A ring final circuit however offers some safety benefit over radial re 2 paths to earth. A radial final circuit of 20A offers some safety benefit over ring in that a double 13A socket can actually only supply 19.5A yet has a 32A CPD. Not a problem except in kitchens with few sockets and lots of heavy appliances where Things Can Get Toasty.

Not worth it at all. Fixed appliances have their own DP isolator, then lockoff the relevant MCB/RCBO or main switch).

Reply to
Doctor Drivel

That's even better. So all this bonding cable can be thrown out by buying two RCBOs, one for the shower fan circuit, whatever one that is, and light circuit the bathroom is off, and 15 minutes fitting at the CU. Much better.

Reply to
Doctor Drivel

In my experience the most common problems found in 1970s installations are

There is no main or supplementary bonding, the landing lightswitch is fed off the downstairs lighting circuit and takes it's neutral from the upstairs circuit, high readings on the ring continuity (as you pointed out) and unfortunately bad DIY.

Minor faults are usually no earth sleeving or red sleeving on the lighting circuits and the odd brocken switch or socket.

The main reasons I get asked to change CUs are (in no particular order)

  1. Just bought a house
  2. Rewires
  3. Selling a house
  4. Wants to rent a house
  5. The fuse box has melted etc
  6. Addition works such as electric showers or extensions

An excellent suggestion. Although there is more money in changing a CU than there is in fixing any additional work.

Maybe an alterative quote from someone else would tell you if their standard rate is a fair price.

Adam

Reply to
ARWadsworth

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