are RCD's always safer?

[SparkyTris]

Presuming that the actual nuts-n-bolts installation is done correctly,
are there any circumstances in which protecting final circuits with an RCD would make those final circuits LESS SAFE than if they were protected with only overcurrent devices (ie MCBs or cartridge/rewirable fuse)?

 

[EFLImpudence]

Interesting question.

RCDs are additional protection and so not alternative.

Actually the answer is NO but only if the rest of the 'nuts and bolts' are done correctly.

I'm sure some people will disregard other things because they treat RCDs as though they are a panacea.
That, though, would be the fault of the person and not the RCD.

 

[RF Lighting]

Any circuit that is important to remain live such as life support machines, emergency control systems and other things I can't think of could be less safe if protected with an RCD due to the increased risk of tripping on what could be a faulty but not necessarily unsafe circuit.

 

[JohnW2]

Any circuit that is important to remain live such as life support machines, emergency control systems and other things I can't think of could be less safe if protected with an RCD due to the increased risk of tripping on what could be a faulty but not necessarily unsafe circuit.

Indeed, you just beat me to writing almost the same. In fact, even if the circuit becomes 'unsafe' in some senses because of the fault, allowing vital equipment, particularly life-support equipment etc. (and some machines), to continue operating may be the lesser of the two evils - i.e. on balance the 'safer' option.

There have been some crazy suggestions around as to why RCD protection can be dangerous - analogous to the equally crazy argument that car seat belts are dangerous because they prevent people being thrown clear of a crash. In both cases, there may be the very occasional case, in very particular circumstances, in which presence of an RCD or seatbelt has resulted in a degree of injury (or death) that wouldn't have occurred without it - but those very rare cases are overwhelmed by the great majority in which the opposite is the case.

Kind Regards, John

 

[EFLImpudence]

, in which the presence of an RCD or seatbelt has resulted in a degree of injury (or death) that wouldn't have occurred without it

Do you mean because of an electrical issue (rather than medical)?

 

[JohnW2]

, in which the presence of an RCD or seatbelt has resulted in a degree of injury (or death) that wouldn't have occurred without it

Do you mean because of an electrical issue (rather than medical)?

Well, I might be able to think of some medical reasons if I tried (although I rather doubt it), but that wasn't really what I meant. In the case of seatbelts, it's obviously straightforward - there will be the occasional person who, in the absence of a seatbelt, would have been 'thrown clear', or prevented from being trapped a burning car etc. Those who produce the silly arguments about RCDs are a bit more imaginative than that - for example, postulating that in the case of a continuing shock current just above 30mA a person may have time to contemplate and attempt a 'controlled withdrawal', whereas the very brief period before an RCD operates may make post-shock violent movement or falls (resulting in injury) more likely. Such an unlikely (albeit not impossible) scenario is so rare as to be, IMO, plain daft! ... but you might call that 'medical', anyway.

More generally, they talk about having falls or other accidents due to being plunged into darkness by an RCD trip, which (lighting failure) may well not have happened in the absence of an RCD.

As you have indicated, in the real world by far the greatest danger of having an RCD is that some people will be lurred by its presence into doing dangerous things that they would not contemplate in the absence of ann RCD, in the belief that it is a panacea - but, as you said, one cannot directly blame the RCD for that. It's a bit like people driving more dangerously when they know they have ABS, or other bits of modern technology which aim (but not perfectly/completely) to protect them from themselves!

Kind Regards, John

 

[bernardgreen]

A single RCD for a whole house will make the electrics safer but as many are pointing out it may create other hazards when it trips.

I can only think of one incident where a tripped RCD caused a serious injury and that was the sudden darkness in a kitchen leading to a scald from the faulty kettle which had caused the RCD to trip.

There have been people very frightened when all the power and as a result the heating in the house went off, one old gentleman couldn't phone for help as his only phone was cordless and needed mains power for the base station.

 

[SparkyTris]

Thanks, all.

The question arose from a recent visit to a friend in Northern Sweden. She was living in a remote timber cabin on the edge of a small hamlet, with a TT supply and no local earth - that we could find.
the fuseboard was quite old - looked 1960s in style, with ceramic cylindrical wire fuses (threaded to screw in& out). no earth bar present. It actually put me in mind of the fuseboxes on the Danish seine netter boats that used to come to E coast ports
the sockets around the house were euro type (schuko or sthg I cant think of the name now - circular 2-pin with an earth tab). the earths just werent used.

She (the owner) was concerned about the installation simply being "old".

I was more immediately concerned about what I perceived was a serious fire risk: the house was made of old and extremely dry (thanks to the arctic climate) resinny wood. (spruce I think)

I thought that putting an rcd on the incoming supply before it entered the fusebox might be an efficient emergency measure with regard to fire: OK it wouldnt stop a cable overheating but it would stop any current flowing through resistive wood in a fault-to-house situation. but would a fault-to-house situation be dangerous with no earth path?
(I can see this is the sort of 'panacea' thinking you warned against, I did mean the rcd to go in as an emergency measure pending re-design

with no practical electrical earth, can an rcd be effective?

 

[bernardgreen]

with no practical electrical earth, can an rcd be effective?

While there is no installed earthing system the RCD will be effective when the un-expected and/or un-planned path to earth occurs due to a fault.

An RCD will improve safety but splitting the supply between two would be better provided that lights are split between them. Or a non-maintained emergency lamp that switches on ( to batteries ) when there is no mains supply to it.

 

[SparkyTris]

OK, understood. but with no practical earth? (the ground is frozen solid with a generous covering of snow for much of the year)
this is probably a question for the out-of-UK forum really.

 

[JohnW2]

OK, understood. but with no practical earth? (the ground is frozen solid with a generous covering of snow for much of the year).

As Bernard has said, even without an (intentional) earth in the installation, if current finds some path to earth (e.g. through a human being), an RCD would trip. There surely must be recognised ways of establishing an 'effective earth' in such climates. Provided one could get a long enough earth rod intothe ground (maybe in mid-Summer?!), it would probably remain effective even when the surfaces layers were frozen.

Kind Regards, John

 

[ericmark]

Where loss of supply can cause danger then yes using a RCD could be a problem. For example the Christmas turkey in the oven for say 6 hours with a built in unit could overheat should the power be removed and so there are no cooling fans.

As to if it could overheat enough to cause a fire I would hope not as other things could also cause power failures.

My cousin would be dead if he had been wearing a seat belt when the tanker that hit him squashed the car forcing him into the passenger seat. However for every one like that there are 1000's the other way.

So we do a risk assessment.

So in real terms only when using a RCD on the supply from an UPS could there really be a danger. One would expect there is a reason for using an UPS so it does raise the question as to what protection should be placed on the output?

Since an UPS will still power items after the main supply cable is cut the whole question of earthing must be considered.

Personally I consider if the UPS powers just one item then like with the bathroom shaver socket then there is no real need for earth connection or RCD protection. However where they supply many items then RCD and earth rods have to be considered.

You state nuts and bolts done correctly so lights and sockets not on same RCD so kettle example not valid. Should always be one phone in house not cordless so also that's not valid.

However my house does have two RCD's and some times when one trips it takes the other with it so sockets and lights do trip together. However in centre of house there is an emergency light and there is always some degree of light from outside.

So for when all done correctly only with a UPS can I see where a RCD should maybe be omitted.

I see a problem while on holiday a RCD tripping may cause you to lose a freezer full of food but that is not a danger just an expense and there are auto resetting RCDs if you really want them.

Your story reminds me of the Falklands where on a farm the lady of the house was getting a tingle from the washing machine. I tried to connect the earth wires but it would seem there was a JB in the loft I could not reach. However my action in connecting as many earth wires that I could reach resulted instead of a tingle she was getting quite a shock. OK supply was from a generator not mains.

However our best intentions can back fire where we only do a small part of the work required. I also when earth bonding in a caravan site ended up stopping all the ELCB-v devices from working. So I am a little wary of just jumping in now, and do a full inspection first.

 

[kai]

Is there any particular reason why a particular piece of important equipment like a freezer cannot be fed from its own dedicated RCD supply circuit, with nothing else fed off that circuit???
Does the 17th edition allow dedicated single RCD freezer circuits?

 

[JohnW2]

Is there any particular reason why a particular piece of important equipment like a freezer cannot be fed from its own dedicated RCD supply circuit, with nothing else fed off that circuit??? Does the 17th edition allow dedicated single RCD freezer circuits?

There's no reason on earth why one can't do that - and many people do have things like freezers on dedicated RCBOs or on non-RCD circuits (provided conditions are met). However, as has been eluded to before, if one has a dedicated RCD/RCBO there is still the issue of a fault in the freezer (or freezer circuit) which results in a small L-E leakage which is enough to trip an RCD, despite the fact that the fault, per se, would not prevent satisfactory ongoing function of the freezer. I know it's very rarely done, but I personally feel that if one does have things like freezers on non-RCD circuits, then there is a case for installing an earth leakage (L-N imbalance) alarm - since otherwise one would never be aware of an L-E fault which was not 'bad enough' to cause an OPD to operate. With a TT supply, of course, one has no option but to have every circuit covered by an RCD or RCBO in one way or another.

Kind Regards, John

 

[viewer]

A question;
Firstly, am I right in thinking:
1. that if a RCCD is fitted but the wiring earth is not existant, then the unit would trip if sufficient current went to somewhere other than the neutral.
2. This would be "earth" whether it be a cabin shell, water pipe, the ground or a designed earthing system and it is to this earth that any fault current would flow
3. The unit would therefore trip before a fatal current could harm the person acting as the live-earth link in a fault situation. No protection if person between live and neutral and none for overloading.
4. In an occupied house, the 'lights out' situation could be covered by using one or more 'emergency plug in torches' (about £12 each in UK)
Now the question:
If this cabin has an external radio, TV, satellite aerial pole, grounded telephone lines etc, could you have enough "leakage" to cause high levels of nuisance tripping - or is this possible but unlikely scenario? Just wondering.