RCD wiki + Common Misconceptions about RCDs

[JohnW2]

The current wiki entry on RCDs consists of just two links which are meant to be to IET articles plus a brief explanation by bernardgreen as to why RCDs trip with N-E faults. The two links merely take one to the IET home page, so maybe whoever put them there can mend that?

Following recent mention in a discussion here of the things which DIYers, the general public and electricians do, and do not, understand about RCDs, I am reminded of the many ‘common misconceptions about RCDs’ which I have often mentioned. I therefore thought that it might perhaps be useful if someone added such a ‘common misconceptions’ section to the wiki entry on RCDs. With that in mind, I've had a stab at a draft of what I think it might say, as follows, and I’d be grateful for any thoughts/comments – particularly in relation to (5), which some people may disagree with. The first two are misconceptions commonly held by the general public. The remainder are more technical, and therefore apply to misconceptions held by some DIYers and electricians:

• Common Misconceptions about RCDs
  
  1...Above all, RCDs do NOT by any means guarantee the avoidance of serious injury or death if someone comes in contact with a live conductor. Indeed, an RCD offers NO protection to a person who comes in contact with both live and neutral conductors. If they come in contact with a live conductor and earth (e.g. ‘earthed’ metalwork), an RCD affords some protection, but still no guarantee of immunity from serious injury or even death. Accordingly, the existence of an RCD should never be allowed to lead to complacency or relaxation of fastidious attention to safety considerations when using or working on anything electrical.
  
  2...An RCD will NOT trip if a two-core cable, such as that used to power many garden tools (mowers, strimmers, hedge cutters etc.) is cut. It will, however, afford some, but not total, protection to someone who (foolishly) picks up the live end of the cable after it is cut whilst they are standing on soil or otherwise in contact with earth.
  
  3...A, say, 30mA RCD does NOT limit to 30mA the current which can flow through a person in contact with live and earth. The current which flows through the person depends on circumstances, and may be much higher than 30mA. What the RCD does do is limit the duration of flow of a current of 30mA or greater can flow to a sufficient extent that serious injury/death will often (but not always) be avoided.
  
  4... Similarly, a correctly-installed RCD does NOT limit the voltage difference which can exist between the installation’s ‘earth’ system (CPCs) and true earth during fault conditions (‘touch voltage’) to 50V. Again, all it does is limit the duration of any ‘touch voltage’ greater than 50V (in practice, usually also the duration of lower voltages).
  
  5...Contrary to what some people believe, a ‘TT’ electrical installations does NOT necessarily have to include a 100mA RCD close to its origin. Because of the high ‘earth fault loop impedance’ (EFLI) in TT installations, over-current protective devices such as fuses or miniature circuit breakers (MCBs) will usually not offer any protection against live-earth faults, so that such protection has to rely on RCDs. In the days when some or all of the circuits within an installation did not have their own RCD protection, the practice therefore evolved of having a 100mA RCD close to the origin of the installation. However, if (as is not uncommon these days), all final circuits in the installation have their own 30mA RCD or RCBO protection, then an additional RCD at the origin of the installation is probably not required. Without such an RCD, lack of protection only exists in relation to the tails going to the consumer unit, and the chances of a live-earth fault occurring there is extremely small provided that these tails are (as generally required by DNOs) short. If an additional RCD at the origin of an installation which contains some 30mA RCDs/RCBOs is utilised (e.g. because some of the final circuits are not RCD/RCBO-protected, or for any other reason), it should not only be a 100mA one but should also be of a ‘time-delayed’ type (‘Type S’) to ensure ‘discrimination’ between it and the 30mA RCDs/RCBOs (i.e. so it will not trip if a 30mA RCD/RCBO trips first), and must be installed in an insulated enclosure. Note that, in relation to any circuits protected by single-pole RCBOs in the consumer unit, this discrimination will only exist in relation to live-earth faults, not neutral-earth ones.

Thoughts? Any ‘misconceptions’ I have forgotten?

Kind Regards, John.

 

[westie101]

I would suggest

2.. An RCD, in most cases.......

I have cut the cable to my mower and tripped the RCD, another time it didn't!

 

[ban-all-sheds]

2) is a jolly good reason, IMO, to use a 3-core flex to supply a Class II portable appliance. Slice through it and you're almost certain to generate an earth fault on the way.

When you expand the article, John, you can correct those links.

Would also be good to explain the difference (with photos) between an MCB/RCD/RCBO, and to warn about old VOELCBs, and of the need for regular testing of RCDs.

Are there time-current curves for RCDs, which would show pictorially what sorts of currents might flow?

 

[JohnW2]

I would suggest
2.. An RCD, in most cases.......
I have cut the cable to my mower and tripped the RCD, another time it didn't!

Westie, you disappoint me - has all that safety training not sunk in? You can be trusted with 11kV but not with a mower?
(as for confessions, I haven't done it with a mower, but I have with a hedge cutter!)

To be serious, thanks - and I agree that those extra words words make sense. Particularly in the case of a mower, I guess there's a possibilty that there will be contact between blade/cable and soil at the moment of the incident!

What do you have to say, from the DNO perspective, about my (5)?

Kind Regards, John.

 

[JohnW2]

2) is a jolly good reason, IMO, to use a 3-core flex to supply a Class II portable appliance. Slice through it and you're almost certain to generate an earth fault on the way.

I agree totally, but I don't think that's within the scope of a discussion about 'misconceptions'.

When you expand the article, John, you can correct those links.

I can certainly try, but I'm not sure what's wrong. The links in the wiki 'look OK', but take one to the IET home page. I wonder if the intended target articles still exist (and still have the same urls)?

Would also be good to explain the difference (with photos) between an MCB/RCD/RCBO, and to warn about old VOELCBs, and of the need for regular testing of RCDs.
Are there time-current curves for RCDs, which would show pictorially what sorts of currents might flow?

All interesting and worthwhile ideas but, again, probably not within the scope of a 'misconceptions' section - so maybe a separate project. Mind you, I wonder how much of that is covered in the IET articles whose links currently don't work?

Any thoughts about my (5) - which I thought was possibly going to be controversial?

Kind Regards, John.

 

[ricicle]

EDIT

 

[JohnW2]

6. An RCD will not operate at the required L to E milli-amp current flow if there is a N to E fault on the circuit creating leakage.

Now that's a subtle one, which I don't think I've ever thought of before!

Just to be sure, I take it your talking about the situation in which there were simultaneous N-E and L-E faults, such that the resultant imbalance of currents in the RCD due to the L-E fault would be reduced?

That's an interesting point, although I think it's only going to be an issue in practice in very rare situations. Are you suggesting that there it should somehow be mentioned in a 'misconceptions' article and, if so, how? Maybe a 'misconception' that an RCD will always operate if there is an L-E leakage of the rated value of the RCD?

There is, of course, a converse - that since there will often be some 'normal' L-E leakage current, an additional L-E current (e.g. through a human body) will often cause an RCD to operate when that 'additional L-E current' is appreciably less than the rated value of the RCD. Do you think that should be mentioned as well?


Kind Regards, John.
Edit: ricicle - I see you have deleted your post since I responded to it. Have you ahd some second thoughts about it?

 

[ricicle]

It is something which I thought might occur but had never investigated. On a sketch I have just done my statement is not true. A N-E fault will not affect the operation - in fact it will make the RCD more sensitive.

Most 30mA RCDs operate around the 25mA mark. Say we have a circuit with 1A flowing through the RCD L conductor. 15mA is flowing N-E so we have 0.985A flowing through the RCD N conductor.
Now someone receives a 10mA shock from the L conductor, only 0.975A flows through the RCD N conductor thus causing operation (25mA imbalance) with only a 10mA shock current.
I am in no way advocating N-E faults btw

 

[westie101]

You can be trusted with 11kV but not with a mower?

And in the second case I was wondering why I was getting "tickles" from the flex as I hadn't spotted it

What do you have to say, from the DNO perspective, about my (5)?

From our perspective it falls outside the scope of responsibility (we'll get rid of all the voltage operated devices our fore-bearers installed one of these days)
For a technical view point I can't see an issue as long as protection is available

 

[JohnW2]

It is something which I thought might occur but had never investigated. On a sketch I have just done my statement is not true. A N-E fault will not affect the operation - in fact it will make the RCD more sensitive.
Most 30mA RCDs operate around the 25mA mark. Say we have a circuit with 1A flowing through the RCD L conductor. 15mA is flowing N-E so we have 0.985A flowing through the RCD N conductor.
Now someone receives a 10mA shock from the L conductor, only 0.975A flows through the RCD N conductor thus causing operation (25mA imbalance) with only a 10mA shock current.

Hmmm - I think you're right in bottom line, but not in your logic. The 10mA 'L-E 'shock current' will increase the current through L of the RCD to 1.01 A, but I see no reason why the N current through the RCD (whatever it's due to) should change - that remains at 0.985A. However, that's still a 25mA imbalance.

If you want to avoid confusing folk, I'd be happy to delete this message and my previous one, if you want to dete yours in between?

Kind Regards, John.

 

[JohnW2]

What do you have to say, from the DNO perspective, about my (5)?

From our perspective it falls outside the scope of responsibility (we'll get rid of all the voltage operated devices our fore-bearers installed one of these days). For a technical view point I can't see an issue as long as protection is available

So does that mean that (as I sort-of suggested), you would be happy with the connection between meter and CU in a TT installation to effectively have no protection against L-E faults, provided it was short?

Kind Regards, John.

 

[westie101]

So does that mean that (as I sort-of suggested), you would be happy with the connection between meter and CU in a TT installation to effectively have no protection against L-E faults, provided it was short?

Let's say that there are no requirements to have the cut-out/meter/CU tails of any shorter length (3m overall) than for any other form of earthing system or for us to use anything other than the cut-out fuse to protect them.
(admittedly for a L - E fault it probably won't, but that would generally only occur if a metal CU were used.)

 

[Paul_C]

With TT, there is the possibility of a fault on the incoming line to the casing of the unit, which obviously would not be detected by any RCD inside the unit since the fault is "upstream" of any such device. Such a fault would then not only make the casing live, but also everything else connected to it throughout the house. Hence the suggestion that with such an arrangement only a plastic casing be used. You might want to work something about that into the piece.

 

[JohnW2]

Let's say that there are no requirements to have the cut-out/meter/CU tails of any shorter length (3m overall) than for any other form of earthing system or for us to use anything other than the cut-out fuse to protect them. (admittedly for a L - E fault it probably won't, but that would generally only occur if a metal CU were used.)

That last point was the one I was making. With TT, a L-E fault occurring in those tails won't cause any protective device (not even the cut-out fuse) to operate but, by the same token, the cables cannot be at any risk - so, particularly given how unlikely such an L-E fault would be (nail through a visually obvious L tail into an underlying water pipe? ), I don't think I see any issues in terms of cable protection. However, the point Paul_C makes about the whole of the installation's 'earth' system then becoming live (without any protective devices operating) perhaps needs to me mentioned/addressed.

Kind Regards, John.

 

[westie101]

I can't help but think that we sometimes lose sight of the practicalities of safety. The phrase "reasonably practicable" is very common in safety terms (look it up it does have a legal definition). I rough terms it means that it is accepted that going to the far end of a fart to mitigate all risks is not really possible and that we have to accept a degree of risk.

I see what you mean about the nail and the water pipe, but in reality what are the chances of that? So I would suggest it is fair to accept these situations.
The use of plastic CUs is an easy one as they are readily available