Wiki: CU

Another one to mull over....

NT

The CU, Consumer Unit or Fusebox provides overcurrent protection for all a house's electical circuits. It usually contains a switch to control all the circuits, and many also feature leakage current protection with [[RCD]] or [[RCD|RCBO]]s.

==Split Load CU== A split load CU has 2 halves with separate power switches. So some of your wiring can be turned off while the rest stays on. So whatever may happen with your electrical system, you don't need to turn it all off. This enables you to have power in the event of a fault, and to have power while working on the installation itself. (There are exceptions of course, such as lightning strike or a major fire.)

Split load CUs are common today (especially with wiring compliant with the 17th edition of the wiring [[regs]]), and can be recommended in nearly all cases.

==Upgrades== Its often assumed that, being newer, [[MCB]]s provide better safety than [[fuse]]s, but the level of protection both provide is very similar, and both have minor pros and cons.

The main ways in which CUs can be upgraded are:

  • Fitting a split load unit
  • Elimination of touchable live parts, as found
** on many CUs when fuses are pulled, ** always live parts are common on pre-war fuseboxes, though these aren't often seen now
  • Incorporation of leakage protection
** with [[RCD]](s) ** with circuit discrimination using RCBOs
  • Increasing the number of circuits rather than extending existing ones
  • Adding provision for a shower, cooker, or other high current load
  • Providing a separate feed for an outbuilding
  • Replacement of 3kA retrofit MCBs with 6kA [[MCB]]s.
  • Replacement of an inadequate, damaged or faulty CU
  • Making an old installation look modern

===Upgrading CUs on TT installs=== A little care is required over earth leakage protection when replacing a CU on a [[Earthing Types|TT install]], as a CU setup not specifically suited to TT use is likely to leave the system dangerous.

==MCBs & Fuses==

===Ratings===

  • Lighting: Usually 6A MCB or 5A fuse. 10A circuits are sometimes used
  • Immersion heater: 15/16A
  • Socket circuits, ring: 30A fuse, 32A MCB
  • Socket circuits, radial: 20A
  • Cooker, shower: Usually 45A or more
  • Small loads: 2.5A, 5A, 6A.

For information on type B, C and D MCBs, see [[MCB]]

==Live parts== A lot of fuseboxes/CUs are in use that expose the user to live parts if fuses are pulled with the power on. Always turn power off before pulling a [[fuse]].

===Working on the CU=== The switch in the CU is the main power switch for most houses. Thus exposed wiring will be live when the CU cover is removed, even with the switch off. Removing the main supply fuse before the meter solves this, but is illlegal, but its common practice nonetheless. Power companies have so far not objected to the practice, for an assortment of legal, safety and financial reasons.

==Trips== Tripping or repeated fuse blowing are normally caused by a fault outside of the CU itself.

  • RCD trips are usually caused by a faulty appliance or a less than ideal installation design. See [[RCD]]
  • [[MCB]] trips are most commonly caused by blowing filament bulbs. Faulty appliances often also cause this.
  • RCBO trips may be due to either of the causes above, since an RCBO combines the functions of both MCB and RCD.

There are of course other possible causes of trips and blown fuses, the above is just a pointer to the most likely.

==Brands== Ashley

Contactum

MK

Wickes

Wylex

  • Cheap & basic Wylex fuseboxes are very common in existing installs
  • Fuses expose live parts if pulled with power on
  • Overheating occasionally happens on high current circuit fuses (greater than 30A)

==See Also==

  • [[House Wiring for Beginners]]
  • [[Rewiring Tips]]
  • [[:Category:Electrical|Electrical articles]]
  • [[Special:Allpages|Wiki Contents]]
  • [[Special:Categories|Wiki Subject Categories]]

[[Category:Electrical]]

Reply to
meow2222
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I think much of the intro and theory of this is already covered in the intro for:

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The CU, Consumer Unit or Fusebox provides overcurrent protection for

Always provides fault current protection, and usually (but not always) provides overcurrent protection.

2 or more sections - 3 and 4 are now quite common.

and some not so minor ones (like unprotected spurs when using imperial spec T&E with re-wireable fuses).

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TT suitable is more down to what you fit in it (or before it) rather than the CU itself. (having said that, one ought to use insulated CUs for TT and not metal clad ones)

I would be seriously worried if that were not the case!

Never seen this - but it may be a phenomenon that only presents with particularly stiff supplies.

I have a feeling it might be worth merging the front of this article with that on the changing a CU one, and making it an intro to CUs article.

The changing a CU one could then just focus on the how and link to the intro. This article could focus on the equipment specifics and also link to the same into.

This section could be expanded into a quite substantial and useful "body of experience" type article. I.e. not only the characteristics and pros/cons of each CU brand, but also cover topics like interchangeability of parts, and servicing older CUs where bespoke parts are no longer available.

Reply to
John Rumm

fairy nuff

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>>> >

Hence why it says 'CU setup'. Some are unsuitable however you configure them.

RCDs fail

I'm surprised. A 30/32A MCB often blows quicker than a 13A fuse.

Good point. Better to leave it there for the moment, until someone decides to do that, or make it an almost empty article of its own?

NT

Reply to
meow2222

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John has I think picked up on the points I was concerned about.

The four that I noted was

your comment on split CU's (yes, 3 or 4 banks rather than just 2) having separate power switches - this is incorrect as I have 3 split CU's (one in the house and 1 in each outside area) and none have separate switches.

You need to explain why TT earth requires a special CU - I'm not sure that is correct either as my understanding is that as long as everything is covered by an RCD/RCBO, nothing more is required.

Referring back to my first comment, somewhere near the end you mention switching off the CU, but if it has several switches you should indicate which one as you just say 'Switch off the CU...'

Beware of words like 'usually' - an RCD trip for instance is *always* caused by a faulty device or faulty wiring, with the only unlikely exception being a faulty RCD. I trust that the RCD article does strongly advise a full investigation by a professional if the cause of the tripping isn't immediately obvious.

Rob

Reply to
Rob G

snip

then in what sense are they split? Or are you saying an RCD isnt a switch?

Again it says 'CU setup' An old Wylex fusebox with no rcd provision is not suitable to replace a broken modern box TT CUs should be plastic And a single RCD only is not compliant

So its not always

Perhaps you could explain why if you do it for yourself you'd become incompetent.

FWIW most causes of RCD trips can be found perfectly safely by a novice with =A35 of equipment.

NT

Reply to
meow2222

Perhaps, but you were talking about fuses (and by association MCBs). A fuse blowing repeatedly due to a fault within the CU would be worrying.

Thinking about it - I have seen it once, but that was in a commercial premises and not a domestic situation, (where the offending fault was a pair of pliers across the supply inside a 4 way trailing lead!)

Only if there is sufficient fault current. Easy enough to achieve on a lighting circuit, but not always so easy on a power circuit at the appliance end of a flex (where you are going to need north of 160A to get into the instant trip part of the curve on a type B MCB).

Having said that, we are talking about an event that is so rare for most people as to be hardly worth mentioning. I am not sure if I have ever needed to replace a 13A fuse in a plug. (More common if you have a supply voltage on the high side I understand).

In the interest of attempting to avoid too much repetition, that is what this article could be the practical advice on CUs one - pretty much all of the other stuff is already included in the numerous other articles on MCBs, Changing CUs, Basic Electrics etc.

Reply to
John Rumm

I've had two 30/32A trips with washing machines where the mcb tripped rather than a 13A fuse. (different washing machines, and different MCBs. Dead short in each case.

I've also had a pair of fuses (80A and 100A in series) fail when a 5A mcb hasn't tripped. This was an early wylex or crabtree MCB where a lightning induced fault caused a dead short, somehere within 10metres of the substation transformer. The first replacement (80A) fuse blew in the hand of the leccy board peep, who was decidedly not amused.

Reply to
<me9

snip

Hmm, OK that is correct in that case but I think that needs clarifying as I would regard myself as competent (electronic engineer and several full house re-wires) and if I'd followed that to the letter I would have expected to find a further switch that looks like the incomer isolator switch.

Fine then add that I suggest.

That's being pedantic as you know full well what I'm getting at.

Hmm, again. I'm not sure that the collective will agree with that. How many times has there been a relatively basic electrical enquiry here and someone like yourself or John R has made the point that if the OP is asking such questions he shouldn't be anywhere near electricity (actually usually said a little kinder !).

You know as well as I do that the common cause of an RCD trip is detectable with the Mark 1 Eyeball - ie every time you switch on the cooker, washing machine, dish washer, and so on, the RCD trips - therefore it's that piece of equipment.

If it's not that then how is a novice with a =A35 DVM from Maplin going to locate the fault ? If he's a novice he won't really know about DVM's and is equally unlikely to know about the niceties of house wiring, nor I would be concerned, the dangers.

Hence my comment.

Rob

Reply to
Rob G

Can I add:

==Brands== ...

Contactum

  • Beware the flimsy busbar cover

Hager

  • Wider than usual range of MCBs and RCBOs, including double pole.
  • Snappable busbar with good quality slip on sleeve makes live working safer and allows custom split load configurations to be made
  • MCBs and RCBOs comptable across the domestic plastic and inductrial metal enclosures.
  • Double pole (SPSN) and single pole devices may be mixed freely on a standard single phase busbar (insulated busbar way on the neutral side and flying lead for the neutral). Double pole devices are general double width.

MEM

  • Expensive, but RCD "pods" are available for some of the MCBs allowing many custom RCBOs to be constructed.
  • Probably the only manufacturer listing a true double pole busbar carrying both phase and neutral on a split comb arrangement, avoiding the need for "flying neutrals".

Square-D

  • Innovative enclosed and hidden busbar which allows devices to be clipped in without removing the busbar from all devices.

Cheers

Tim

Reply to
Tim S

I'm not too sure that it's a good idea comparing brands on something like Wiki; it might start to appear that Wiki is 'pushing' a particular make. By all means point out factual differences (features etc.), but leave out opinions...

Reply to
Frank Erskine

Frank Erskine coughed up some electrons that declared:

Fair enough. So remove the reference to Contactum then? I think the rest were fairly matter of fact, but if you saw anything else, please do point it out.

BTW - my obsession with double pole should be explained: I like it on external circuits, though it's not generally required, except with a TT system, in which case I *think* its is mandatory, though I'd have to check that to find an explicit regulation.

I may be making my outside circuits TT, so I hedged my bets a bit - anyway, having double pole isolation on shed and garden lights feels right IMHO, whether it's required or not.

Cheers

Tim

Reply to
Tim S

Editable version now live:

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free to add your comments on any brand you have used...

Reply to
John Rumm

Tim My house is TT so I'm interested in what you are saying - but what has to be 'double pole' ? And why?

And also, why are you making your outside circuits TT ? Is this to avoid the voltage rise in the return to the CU with an inadequate CPC, or questionable earthing in the casing if using SWA ? I assume that the outside circuits are covered by an RCB at the point of use.

Thanks Rob

Reply to
Rob G

John Rumm coughed up some electrons that declared:

Looks good John. I'm sure MK warrents some description as they are widely used and sold. But having not personally touched on, I don't feel qualified. Anyone?

Cheers

Tim

Reply to
Tim S

ok, hopefully its now clearer

done

That's criticism of a signifcantly different scenario to using a multimeter on all plug-in appliances to look for a leaky one. The latter is very safe.

...if youre lucky. Often not.

as explained above. Plug-in appliances outnumber fixed ones. The one fixed one that often causes RCD trips is the immersion heater, and those pass so much leakage when they split that they leave little doubt as to which appliance is faulty. Hence often a basic meter applied to plug pins is enough.

NT

Reply to
meow2222

great, cheers

NT

Reply to
meow2222

  1. a lot of editing had already been done, so editing an already out of date version live isnt really constructive.
  2. the article already written is CU rather than Consumer Units. Consumer unit and fusebox were going to be redirects to it. This makes creating links in other articles way less work.
  3. the shifting of material between CU and replacing a cu hasnt even started, and discussion of it has been very limited so far.
  4. Proposing and discussing edits here enables them to be discussed and concensus reached rather than simply anyone editing any way on the wiki with no discussion.

Now with 2 very divergent versions we have extra work to do, its going to be alot easier not to do this next time.

NT

Reply to
meow2222

Rob G coughed up some electrons that declared:

Something I picked up on the IEE/IET forums. I'll see if I can locate anything in the regs or OnSite tomorrow.

It's more to do with the supplier's earth. Although EDF claim, in writing that I have a TN-S system, it measures and looks more like a TN-C-S (which is available as an upgrade in the area anyway, meaning there's a good chance their records are wrong).

I've read various articles in the IEE's "Wiring Matters" magazine that say the it's not desireable to export TN-C-S earths to outbuildings under certain circumstances - but again, I'd have to go digging to see if I can find out where exactly in the regulations it says that, if at all. Certainly, it's better not to export TN-C-S earths to buildings with supply-earthed metalwork, eg a steel garage or greenhouse.

Folklore says that odd faults (including on the supply network) can lead to TN-C-S supply earths gaining significant potential above the local ground. In a house, this doesn't matter much[1] as things are bonded so as to give an equipotential zone or zones internally. But outside, this is less true as it's much more likely that the use could be in contact with the ground.

[1] What happens with a ground floor bathroom with a concrete slab floor on earth? Short of including a metal grid in the floor, bonded locally, there's not much you can do about this.

I'll have RCBO protection back in the house CU. I haven't fully committed to which design to use, and I won't be doing the outside for a while, but I wanted to get a the RCBOs that gave me the most options.

If anyone can add any clarity to the above, I'd like to read it. In the meantime, I'll have a dig around the red book.

Cheers

Tim

Reply to
Tim S

I uploaded and included the edits mentioned so far... (in the brands section anyway - I left out the rest for the moment anyway, but it seems easier to capture things like the interchangeability matrix in situ)

I don't really like having abbreviations as article names - nothing to stop you having CU and fusebox as redirects to it though. Failing that once can always rename it if required.

and?

Tis what the talk page on the wiki is for as well...

most wikis seem to function ok without the backup of a newsgroup.

You worry too much... ;-)

Reply to
John Rumm

Having used lots of contactum, I thought I would fill that out a bit...

I take it you mean double pole RCBOs here?

You get that via the double pole switch or RCD ;-)

(then again I usually stick external stuff on a dedicated CU so it is completely separate from the house one)

Reply to
John Rumm

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