Wiring in 17th C Timber Framed house

[Penners]

So the master switch in my consumer unit which trips out if there's a short anywhere in the house is actually an RCBO and not (as I tend to refer to it) an RCD?

 

[MdB]

So the master switch in my consumer unit which trips out if there's a short anywhere in the house is actually an RCBO and not (as I tend to refer to it) an RCD?

MCB - Miniature Circuit Breaker - Trips on over current. Its rating will be marked on it and it is guaranteed not to trip when that amount of current is going through it. Its actual trip current will be quite a bit higher. No test button.

RCD - Residual Current Device - (Proper name RCCB - Residual-Current Circuit Breaker but I don't hear people using this term) Trips when there is a difference of typically 30mA between Live and Neutral. Will have a test button. The current rating marked is a maximum allowed before it melts. It won't trip if this is exceeded.

RCBO - Residual Current Circuit Breaker with Overload protection - Combines both functions of MCB and RCD so will have both a test button and a current rating at which it won't trip. High currents will cause it to trip.

The single width RCBO can be used to replace an MCB in a fuse box. Mine is half full of them.

 

[Lime]

So the master switch in my consumer unit which trips out if there's a short anywhere in the house is actually an RCBO and not (as I tend to refer to it) an RCD?

To put what MdB said in a nutshell, no.
Almost certainly the switch you describe is an RCD.
It trips out because there is a leakage to earth.

 

[MdB]

So the master switch in my consumer unit which trips out if there's a short anywhere in the house is actually an RCBO and not (as I tend to refer to it) an RCD?

To put what MdB said in a nutshell, no.
Almost certainly the switch you describe is an RCD.
It trips out because there is a leakage to earth.

Rather than a short.

If it has a test switch then it is an RCCB (RCD) or an RCBO (unlikely for the whole house).

<wrong bits deleted>

 

[Penners]

Well thanks to you both for those explanations. I'm not currently ( :wink: ) at home, so I can't examine the breakers in my CU. I know I have an MCB on each individual circuit, but I'll need to check whether my master breaker (which does have a test button) is an RCD or an RCBO.

If it's the former, would you change it for the latter?

 

[MdB]

deleted as some of it I had got wrong....

 

[Flyfisher]

If you don't have your own 80A MCB then you need to have one.

I don't think that's strictly correct, at least not in a 'wiring regs' sense.

We recently had a new CU professionally fitted and certified. It has a mechanical 100A on/offswitch and two 'zones' each protected by a 63A RCCD and each circuit protected by a suitably rated MCB.

It might be helpful to think about the differences between MCBs and RCCD/RCDs.

MCBs are essentially resettable fuses and provide protection for over-current situations. Thus, plug in too many electric fires and kettles etc to a single socket circuit annd the 30A rating of the cables will be exceeded. Obviously this is a bad thing and a potential fire risk so the MCB (fuse) will trip (blow) to prevent this.

RCCD/RCDs protect against fault conditions such as drilling through a cable, running over a cable with an electric lawn-mower or even touching a live wire. They operate with tiny currents and will trip before (theoretically) a fatal electric shock.

Thus, from a safety perspective, a single RCCD/RCD operating across all circuits will provide adequate safety against 'faults'. However, a single MCB (fuse) for the entire CU would not be safe because a single circuit could be easily overloaded (unless 100A wiring was used throughout!), so each circuit requires its own MCB.

But there are other considerations, which are mainly to do with convenience than absolute safety. For example, it's not very convenient for the entire house to be cut-off because of a fault or, in extreme cases, 'nuisance' tripping of RCD/RCCDs (which we used to suffer due, I think, to water leaks - but not in recent years thankfully).

For these reasons, it's quite common for a CU to be split as described above. If there is a fault on any circuit, the relevant RCD/RCCD will trip but the other half of the CU will continue to operate as normal. I suppose the ideal situation is for every circuit to be protected by a combined RCD/MCB device (an RCBO) and I can only assume that this isn't commonly done because of the additional cost. Such a cost might result in slightly less inconvenience when a fault does occur, which is typically very rare, but it won't actually provide any greater protection against shock hazards - thus, a split CU seems a reasonable compromise.

 

[MdB]

I've deleted various bits I had written above as it was misleading / wrong. Sorry people.

 

[Lime]

So the master switch in my consumer unit which trips out if there's a short anywhere in the house is actually an RCBO and not (as I tend to refer to it) an RCD?

To put what MdB said in a nutshell, no.
Almost certainly the switch you describe is an RCD.
It trips out because there is a leakage to earth.

Rather than a short.

Indeed, although a short may occur at the same time.
If there was a short without an earth leakage I'd expect the circuit breaker or fuse protecting that specific circuit to trip/blow, not the main switch.
As Flyfisher alluded to, the short-circuit current required to trip an 80A RCBO or MCB is very high and most final circuits would be damaged if the circuit with the fault were not also protected by a smaller fuse, MCB or RCBO.

If it has a test switch then it is an RCCB (RCD) or an RCBO (unlikely for the whole house).

Agreed.

Not to be confused with the much older ELCB (or VOELCB) that was interposed in the main earth lead to detect voltage (50V) which also had a test button.

 

[Penners]

Thanks, FF, for that added explanation.

I understand the basic principles that have been explained but I'm a little hazier on some of the subtleties. However, bottom line, it sounds as though our installation, with a correctly rated MCB on each circuit and an RCD (I think) protecting the whole CU, is an acceptably safe configuration.

Yes, we do get the occasional nuisance trip of an MCB or the RCD - principally when a light bulb blows - but I can live with that.

 

[Moo]

the occasional nuisance trip of an MCB or the RCD - principally when a light bulb blows

That never bothers me - it's reassurance that the system works.

 

[Penners]

I agree (even though I have a certain amount of trouble convincing the other resident in our house that it's NOT an indication of major, potentially deadly problems!)

 

[Flyfisher]

I understand the basic principles that have been explained but I'm a little hazier on some of the subtleties. However, bottom line, it sounds as though our installation, with a correctly rated MCB on each circuit and an RCD (I think) protecting the whole CU, is an acceptably safe configuration.

I'd say so. It's equivalent to our new installation and I sleep well enough at night.

Yes, we do get the occasional nuisance trip of an MCB or the RCD - principally when a light bulb blows - but I can live with that.

It would be interesting to know if the RCD ever trips when a light bulb blows. I can imagine a blowing bulb causing a momentary over-current surge causing an MCB to trip but I can't imagine one causing an RCD to trip because they only have the live & neutral conductors within the glass so any current cannot 'leak' away to cause the required imbalance to trip an RCD. Unless they can be susceptible to a current surge?

But yes, I agree, the occasional RCD trip is mildly reassuring though the 'golden rule' of fuses still applies: if it blows/trips once, simply reset/replace but if it immediately blows/trips a second time then investigate why (or contact someone who can).

 

[MdB]

I've never understood why blowing bulbs can trip RCDs but they do. Capacitive coupling down the earth wire ?? (I don't believe that)

Having one RCD for the whole house isn't recommended now and wouldn't (??) be done for a new installation. 16th addition (I think) recommends splitting the house in two with to RCDs so that half of the lights are on the one RCD and half on the other. This way when one trips, there is still lighting in the building somewhere. 17th addition (I think) takes this further and suggests using RCBOs so that you lose on a single circuit when a trip occurs. It is certainly sensible to have the freezer on a different RCD / RCBO to everything else.

The above might not be absolutely correct in the details but you get the idea.

 

[Flyfisher]

Having one RCD for the whole house isn't recommended now and wouldn't (??) be done for a new installation. 16th addition (I think) recommends splitting the house in two with to RCDs so that half of the lights are on the one RCD and half on the other. This way when one trips, there is still lighting in the building somewhere. 17th addition (I think) takes this further and suggests using RCBOs so that you lose on a single circuit when a trip occurs. It is certainly sensible to have the freezer on a different RCD / RCBO to everything else.

The above might not be absolutely correct in the details but you get the idea.

I didn't realise it was actually in the wiring regs but it certainly makes makes sense and is probably why my electrician recommended the split CU.

I wonder how long it will be before RCBOs on every circuit becomes standard?

 

[FamilyWiggs]

I wonder how long it will be before RCBOs on every circuit becomes standard?

Bit of unecessary expense isn't it?

A single RCD protrotecting all the circuits and detecting earth leakage, with individual MCBs protecting each circuit from overcurrent seems adequate.

 

[MdB]

Bit of unecessary expense isn't it?

A single RCD protrotecting all the circuits and detecting earth leakage, with individual MCBs protecting each circuit from overcurrent seems adequate.

They are around £15 each if you shop around so not too bad. I think one for the freezer by itself is essential. I also have one for the landing light and the mains powered smoke alarms. This way if the rest of the house trips there is still light available to most of the rest of the house and the smoke alarm is still active. If it trips itself, the landing light won't work which will warn me that the smoke alarms are out. The rest I guess are less essential but nice if you can spare the few £ extra.

 

[Flyfisher]

Bit of unecessary expense isn't it?

A single RCD protrotecting all the circuits and detecting earth leakage, with individual MCBs protecting each circuit from overcurrent seems adequate.

Unnecessary in terms of improving electrical safety, I agree, but useful in terms of convenience, as MdB described. Although it might also be argued that it's safer for half the lights to remain on if an RCD trips.

 

[FamilyWiggs]

I guess it depends on your CU. Mine has a built in RCD covering all the power circuits, so I don't see the convenience value of adding in 10 further circuit specific RCDs at £15 a go. I'll bet the main RCD trips as well as individual ones!

When the main RCD trips, I switch off each MCB before resetting the RCD, and then reset each MCB in turn until the RCD trips again, thereby identifying the problem circuit.

I can certainly think of other areas needing £150 first...

 

[Penners]

It would be interesting to know if the RCD ever trips when a light bulb blows. I can imagine a blowing bulb causing a momentary over-current surge causing an MCB to trip but I can't imagine one causing an RCD to trip because they only have the live & neutral conductors within the glass so any current cannot 'leak' away to cause the required imbalance to trip an RCD. Unless they can be susceptible to a current surge?

Thanks for the reassurance, FF.

Yes, a blown lamp does occasionally trip the RCD. I expect that your and MdB's explanations for this are probably correct, although I still carry a slight suspicion that it's due to the premature discombobulation of the left-handed snib valve agitator matrix.