are RCD's always safer?

[bernardgreen]

1 The RCD will trip if the currents in the live and neutral differ by more than the rated trip current. A borrowed neutral can result in an RCD tripping/

2. It need not be an earth.

3. 30 mA or less can be fatal. RCD do NOT prevent lethal shocks, they reduce the risk of death.

4. agreed.

 

[JohnW2]

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.

Exactly. That's what I said. Of course, if there is no 'intentional' or 'designed' earth to the electrical installation, there will probably be less potential paths to earth within the cabin - only those other 'incidental'paths you mention. As for the bit I've highlighted in red above, an RCD may achieve that (such is the hope) - but there is no guarantee.

No protection if person between live and neutral and none for overloading.

Sure, but that's true of any RCD, in any situation.

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)

That would help, but unless you had dozens of them, all over the house, there would still be a risk. Imagine that you had just lifted a very heavy pan of boiling water off the hob when everything went pitch dark!

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.

I don't really understand this question. As above, you clearly understand that an RCD will trip if there is a sufficient (usually >30mA) leakage from live to earth - why should the presence of any of those things increase the chance of that happening? As above, any of those things might be connected to earth, so could form a 'shock path' to earth if someone touched one of them at the same time as a live conductor (even with no 'designed earth' in the cabin), but their mere presence cannot really result in RCD tripping.

Kind Regards, John

 

[viewer]

Many thanks. Always useful to get a view from someone with knowledge and a different - real life - perspective. For the aerials etc I was musing (without knowledge) about a leakage to the real earth (due to the equipment in use)via the aerial lead/metalwork which would not be dangerous but might cause RCCDs to trip. Anyway, thanks for comments

 

[JohnW2]

Many thanks. Always useful to get a view from someone with knowledge and a different - real life - perspective. For the aerials etc I was musing (without knowledge) about a leakage to the real earth (due to the equipment in use)via the aerial lead/metalwork which would not be dangerous but might cause RCCDs to trip. Anyway, thanks for comments

As you know, for the RCD to trip, you need a 'leak' from live to something (usually earth, whether 'designed' or not). Some items of equipment (mainly electronic, usually due to power filtering) do result in very small live-earth leaks, such thats several of them on the same RCD may result in trips - but that's exactly the same whether the leae is to a 'real designed earth' or an 'incidental' one - it is not more likely with the latter.

Kind Regards, John.

 

[bernardgreen]

Do not forget that in some situations a leak from Neutral to earth will also trip an RCD

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

In a system with no earth ( CPC ) a Neutral to earth ( ground ) fault would trip an RCD if the voltage on the neutral was not the same as the voltage of the ground.

 

[viewer]

Thanks again. My thinking was that the OP might from those factors be able to make a rough risk assessment. So in an imaginary scenario, the risk of electrocution is X p.a. and the risk from an accident due to lights failure is also X p.a. therefore there is neither advantage nor disadvantage in fitting a RCCB. Normally in UK we'd expect the accident risk figure to be lower than the electrocution risk figure and have a very low number of nuisance trips. Please ignore semantics and also accept that a 'mild' shock could cause spillage of the boiling water and this would be the case with or without a RCCB.
There is, of course, no way to put numbers to the idea for a single cabin, but nationally there may be useful data.
Now the risk of electrocution with sound wiring/appliances should remain fixed, but the specifics for the OP's scenario could mean a move to loading the accident side of the equation. Thoughts are; Sweden=long nights; old lady more likely to be infirm; cooking and heating on woodburning stove & etc. The reason for asking about "leakage" was that the original assumption on the (low) number of nuisance trips would not hold in a situation where there was "excessive leakage" and I was wondering about an old person with old radios, TVs, etc, being in a forest and having a possibly powered aerial mounted on a pole - etc. If this held true, then the number of nuisance trips during the hours of darkness would be much higher than that experienced in an average UK house and the accident risk would be correspondingly multiplied (and may overwhelm the risk of electrocution).
It was a thought experiment and without data, but I was wondering if the OP could make a SWOT analysis by looking at the factors and using help from posters.
Thanks again.

 

[JohnW2]

Thanks again. My thinking was that the OP might from those factors be able to make a rough risk assessment. So in an imaginary scenario, the risk of electrocution is X p.a. and the risk from an accident due to lights failure is also X p.a. therefore there is neither advantage nor disadvantage in fitting a RCCB. Normally in UK we'd expect the accident risk figure to be lower than the electrocution risk figure and have a very low number of nuisance trips ... Now the risk of electrocution with sound wiring/appliances should remain fixed, but the specifics for the OP's scenario could mean a move to loading the accident side of the equation.

The logic of your probabilistic risk assessment is totally correct. The risks on the 'anti-RCD' side of the equation really all depend upon the hypothetical consequences of failure of lights or some types of equipment and, although we obvioulsy don't have any hard data, I think it very unlikely that these would get anywhere near cancelling the 'pro-RCD' side of the equation, even in a Swedish cabin.

One of the problems is that, on both sides of the equation, the risks (of death or serious injury) are incredibly low, frustrating attempts to get any hard data on which to base such an assessment. Even in the pre-RCD days, fatal electric shocks were amazingly rare. In our days of RCDs, it would be next-to-impossible to get any data on the frequency of RCD trips in situations which otherwise might have resulted in serious injury or death.

Kind Regards, John

 

[securespark]

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.

Dead right, mate.

Like the folk who think that, as long as the Zs is under 1667, they can whack an RCD on the circuit and everything'll be hunky dory.

 

[EFLImpudence]

Yes, totally agree.

One of my pet hates (I have many) which I just don't understand.

Even my automatic certificate programme enters 1667 for everything if there is an RCD anywhere near.

I have to manually change them back to what they should be.