I found that the most helpful yet, having grown quite old trying to read this thread.Hmmm! I asked for help...
Maybe you'd be better off with Bas's book recommendation.
MPB - regs or common sense?
- Thread starter JohnW2
- Start date
Maybe you'd be better off with Bas's book recommendation.
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Bonding the CPC to pipework that is connected to metallic pipework that has contact with the ground DOES compromise the effectiveness of ELCB protection.
The CPC in a TT with ELCB is NOT a protective earth, it is a SIGNAL lead to the ELCB which causes the ELCB to trip if the signal on the CPC exceeds approx 50volts.
JohnW2's diagram is basically correct but the full circuit "in the ground" is far more complex and is different for every installation.
But it was generally accepted that the impedance to ground from a metallic water service pipe was lower than the impedance to ground of a normal earth rod electrode. The water pipe is longer, has a larger circumference and is at a greater depth than the ground rod and hence has a greater surface area in contact with soil that is likely to be damper and more conductive than the soil in contact with the ground rod.
If the fault is a human body then the fault current onto the CPC signal wire has to be enough to operate the coil in the ELCB ( electromagnets are current operated even if they are specified by giving a voltage at which they will operate ). With no bonding of the CPC to pipe work all the fault current has to pass through the ELCB and this was specified as that current which resulted in 50 volts appearing across the coil and a current considered "not fatal" in the human body. Specifying the operating point as a voltage instead of as current made it far easy to measure. To measure current would require disconnecting the CPC from the F terminal and inserting a milli-amp meter into the circuit.
Any alternative path for some of the fault current to reach ground will require the fault current through the body to be increased if the current through the ELCB coil is going to be enough to operate the ELCB.
Thanks. That has, of course, been my contention all along (seems more-or-less common sense to me), but people here have suggested otherwise, and I've also seen that claim written in many other places.
Exactly - and that was obviously the basis of my calculations. Of course, the CPC can become much more of a protective earth if the supply pipes are bonded (explicitly or implicitly)
Yes, of course - my diagram was obviously very much simplified for the exercise.
Exactly. I took the conservative approach of assuming that RE1=RE2 (so that I couldn't be accused of using an extreme example) but I agree the reality is very likely to be that RE2 (pipework) will be lower - in which case the situation (compromising of the ELCB's function) gets worse. Even with my example, only about 5% of a fault current goes through the ELCB; with lower RE2 (water pipe impedance to ground), that would become even less.
Again, my very points. With my conservative example, adding bonded pipework would mean that the current through te human body required to trip the ELCB would have to increase about 20-fold, and that factor would increase if the impedance to earth via the pipework was lower; we almost certainly would be in the range of 'fatal' currents. Given the high probability of implicit, if not also explicit, bonding to supply pipes, I can't help but wonder whether these devices ever saved any lives at all!
So, as far as you are concerned, it doesn't seem that I am missing anything at all - so I gues I have to await any comments from those who feel that my (and your) conclusions are incorrect.
Kind Regards, John
There will inevitably be threads which individuals find uninteresting, tedious, or worse, and one expects that they will not persist in trying to read them.
If I get a chance, I may just do that. However, assuming that it basically agrees with the point I've been making, that will do nothing to alter the situation regarding those here who seem to think otherwise.
John
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P
Paul_C
I think when it comes to talk about whether or not the effectiveness of the ELCB is compromised in this situation, it depends exactly upon what is meant by that, and this is where we're at crossed purposes.
If looking from the perspective of the level of earth-fault current required to trip the ELCB, then yes, I agree that extraneous paths to earth on the "F" side of the coil will result in a higher fault current being necessary. I think that's the way you're looking at the situation, comparing it with modern RCD protection.
But these devices are voltage-operated ELCB's, not current-operated types. The intent of them was not to trip at some specific level of fault current, but to trip to prevent the potential on the installation's protective conductors from rising above a level deemed safe, i.e. about 50V.
In that respect, then so long as the reference electrode is correctly located outside the influence of other earth connections (intentional or otherwise), then the ELCB does the job it was designed to do, i.e. its effectiveness in preventing the voltage on its "F" side from rising above 50V is not compromised. It just takes a larger fault current to trip the ELCB, due to that resistance to earth on the installation side of the coil.
So does a stray earth on the "F" side affect the level of fault current necessary to trip the ELCB? Yes.
Does it compromise the ELCB's intended function of preventing the voltage from rising above 50 volts? No.
If looking from the perspective of the level of earth-fault current required to trip the ELCB, then yes, I agree that extraneous paths to earth on the "F" side of the coil will result in a higher fault current being necessary. I think that's the way you're looking at the situation, comparing it with modern RCD protection.
But these devices are voltage-operated ELCB's, not current-operated types. The intent of them was not to trip at some specific level of fault current, but to trip to prevent the potential on the installation's protective conductors from rising above a level deemed safe, i.e. about 50V.
In that respect, then so long as the reference electrode is correctly located outside the influence of other earth connections (intentional or otherwise), then the ELCB does the job it was designed to do, i.e. its effectiveness in preventing the voltage on its "F" side from rising above 50V is not compromised. It just takes a larger fault current to trip the ELCB, due to that resistance to earth on the installation side of the coil.
So does a stray earth on the "F" side affect the level of fault current necessary to trip the ELCB? Yes.
Does it compromise the ELCB's intended function of preventing the voltage from rising above 50 volts? No.
P
Paul_C
Here are a few relevant regulations on ELCB's from the 14th edition:
Note: D.22 sets out the conditions for protection by fuses or excess-current circuit breakers alone.
Note: D.22 sets out the conditions for protection by fuses or excess-current circuit breakers alone.
Yes, that appears to be the case.
Indeed it is how I'm looking at it. The adage I first learned in the 60s (when I was sometimes playing with much higher voltages than we are talking about here!) that "It's volts that jolts but mils that kills" remains true.
Indeed so, but, at least with modern knowledge, I am not totally sure what that was meant to achieve. If a human being is forming a path between Line and CPC (aka 'F' terminal of ELCB), the potential difference across them (which is obviously a primary determinant of the current passing through them) will be VLINE - VCPC (or VLINE-V'F'). In fact, and rather ironically, the potential difference across the victim will decrease as the voltage being sensed by the ELCB (i.e at the 'F' terminal) increases.
I can but presume that these devices were never really expected/intended to protect a victim against the effects of their becoming a LINE-CPC path - but, rather, the devices sought to limit the potential of anything connected to the CPC (e.g. what we would today call 'exposed conductive parts' to a voltage relative to earth that was deemed to be safe.
Indeed - but, getting closer to some of my professions (I have more hats than I care to think about!), this reminds me of an ancient Greek physician (can't remember which one) who became (in)famous for saying something along the lines of "the treatment worked sucessfully, just as intended, but the patient unfortunately died"
It seems that we are now all agreed, and that I wasn't really missing anything other than the fact that we were talking about different flavours of 'compromising the functioning of the ELCB'.
As I wrote recently, I do have to wonder whether these things ever saved many lives!
Kind Regards, John
And since protecting a human being from death by electrocution requires that the current is limited the protection of life is compromised by alternative routes to ground for the fault current bypassing the ELCB coil.
The possibility existed of very high fault currents flowing through the alternative route before the voltage on the CPC reached the trip point of the ELCB. In worse case the over current fuse could blow before the ELCB operated.
The ELCB was a useful device when the CPC could never be connected to any other route to ground. As soon as pipe work from the ground could become connected to the CPC ( boilers and other equipment where pipes and electrical earthing met ) then the ELCB could no longer be relied on.
That's all very true, but ....
I have to wonder whether they were very useful, in terms of saving lives, even in the absence of alternative routes to earth. In that situation, if a part of the human being is 'connected' beween Line and CPC, then the impedance of the human being and the ELCB coil are simply in series, with the same current going through both.
My understanding is that the ELCB coils generally had an impedance in the range 100 - 300 Ω. If that's true, the current passing through the coil (and the human being) when the 50V trip voltage was reached would have been in the range 167 - 500 mA - more than enough to kill, particularly if the path through the person was hand-hand or hand-foot.
Kind Regards, John.
P
Paul_C
The point about the voltage-operated ELCB though is that it was never designed or intended to provide direct protection against electrocution from somebody getting across L & E, any more than the current-operated ELCB of the same era was. It was only intended to be used to provide protection against earth faults where loop impedances were not sufficiently low to allow fuses or excess-current circuit breakers alone to provide such protection. In other words, it protected against indirect contact.
Indeed, as I wrote in my last post 'to' you:
The irony is, of course, that exposed metalwork at a voltage more than 50V above earth would only really have been a hazard if there was some other (presumably 'unintentional') unbonded true earth around for someone to touch at the same time - which brings the discussion more-or-less round in a circle, since the device would only really have protected against a risk of electrocution if there was unbonded pipework in the equation.
Kindf Regards, John
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