Myths

[JohnW2]

So what can be done about the situation for the infinite number of possible fault currents that people seem to think might exist when one stray wire touches something it should not? I feel we must be missing something.

There is obviously nothing 'comprehensive' one can do about it, because fault currents (flowing from L to E) could theoretically have any value - from the 'PEFC' (which is what we have when we assume that the fault itself has zero impedance0 down to values which would be quite credible as 'L currents in normal operation'

One this which cab be (and is) done is to have the 'additional protection of RCDS, since an RCD will detect a tiny ("L-E") fault current.

In terms of ADS using OPDs, what surprises me is that they have not made at least some allowance for the fault impedance being finite (i.e. at least a little above zero) - even if only, say, 0.1Ω or 0.2Ω. If that were done, the tabulated 'maximum Zs' figures we work with would reduce by that amount. Let's face it, even a 'deliberate connection' is, in the real world, not going to have an impedance of 0.0000000...Ω, so it seems totally unrealistic and unreasonable to assume that the 'accidental touching' of two conductive parts will have a contact resistance as low as that.

I have said before I once blue the DNO 60A fuse when a stray T&E CPC happened to spring into the CU main switch line incomer screw. It did not stay there, of course, but was blown back immediately by the flash. Obviously the current must have been great enough for the instant disconnection with no damage to anything apart from a small globule melted on the end of the CPC and a mark on the screw.

Fair enough.

How then do these faults of quite considerable resistance manage to maintain contact for long enough if the disconnection is not 'instantaneous'?

I'm not sure what you mean by "faults of quite considerable resistance". A 1Ω fault would result in 230 A, which would blow a 60A fuse more-or-less 'instantly', wouldn't it?

The problem is that the impedance of the fault is very unpredictable. I've just done a quick experiment gently touching and stroking the end of a copper conductor against a bit of brass, and got resistance readings varying from <0.01Ω to about 5Ω.

Kind Regards, John

 

[JohnW2]

We need to consider are the readings we take with our meters may not be that accuracy. Example of 1.111 of anything on a meter may prompt some to assume it might be accurate to +/- 0.001 , this is far from reality. ... Accuracy is often far worse than resolution might induce some to think it might be. ... Treat meter accuracy on most equipment as a bit better than a length of damp string. It`s only a guide. Two readings seeming far apart might actually be very closely spaced apart. A ball park figure. ... If a reading is well within what we expect it to be or well without what we are expecting might be an indication of whether it looks OK or not.

That's all true, but I don't think it's relevant to what I've been saying, since I am not talking about resistances/impedances anywhere near the limit of accuracy of the test equipment

For a B32 MCB, the tabulated 'maximum Zs' (taking Cmin into account) is 1.37Ω, so I presume that 'we' (and an EICR inspector) would be 'very happy' if the measured Zs was, say, 1.2Ω. However, if, given that Zs, an L-E fault of, say, 0.2Ω were to arise, the fault current would only be about 156 A (at 218.5V), hence not necessarily quite enough to magnetically trip the MCB (which could require 160 A) and hence achieve the required disconnection time.

Kind Regards, John

 

[EFLImpudence]

I'm not sure what you mean by "faults of quite considerable resistance".

The non-negligible impedance ones that you (or others) are worried about.

A 1Ω fault would result in 230 A, which would blow a 60A fuse more-or-less 'instantly', wouldn't it?

Yes, exactly.

The problem is that the impedance of the fault is very unpredictable. I've just done a quick experiment gently touching and stroking the end of a copper conductor against a bit of brass, and got resistance readings varying from <0.01Ω to about 5Ω.

Can you try that with a live wire and see if you can get a "fault" which will take some time to trip the OPD?

 

[ebee]

Can you try that with a live wire and see if you can get a "fault" which will take some time to trip the OPD?

Of course you can but you`d need to take conserable precauitions in this "Don`t try this at home" experiment.
I now expect a few contributors to plot a graph

 

[JohnW2]

The non-negligible impedance ones that you (or others) are worried about.

I'm not 'worried', because I've always accepted it as an inevitable fact of electrical life - and have only been talking about it because ebee raised the issue.

Asa I've said, I just wanted to remind people that they are fooling themselves if they believe that if a circuit has a Zs below the tabulated/calculated 'minimum Zs', then they will necessarily get disconnection within the required time in response to 'real-world' faults.

A 1Ω fault would result in 230 A, which would blow a 60A fuse more-or-less 'instantly', wouldn't it?

Yes, exactly.

I was typing so fast that I forgot the very point I've been discussing (well, the reverse of it), and therefore got that wrong! My apologies.

Just as the total EFLI is not just the measured/calculated "Zs" (because it also includes the impedance of the fault), nor is it just the impedance of the fault (because it also includes the measured "Zs" - actually Zdb, roughly Ze, in your example). Hence, in the worst case, of a TN-S installation with a Ze of 0.8Ω, with a 1Ω fault, the total loop impedance would be 1.8Ω, hence a current of about 128 - which may not be high enough for truly 'instantaneous' blowing of a 60A fuse (a quick glance at tables suggests that it might take around 3-4 seconds - certainly a lot more than the required disconnection time for fault protection).

Can you try that with a live wire and see if you can get a "fault" which will take some time to trip the OPD?

I'll leave that to you - but, as has been said, I really think you should have included some strong 'don't try this at home' warning.

Kind Regards, John

 

[ebee]

have only been talking about it because ebee raised the issue.

Sorry, have I caused some controvesy? (not like me is it? )

 

[JohnW2]

Sorry, have I caused some controvesy? (not like me is it? )

No need to apologise - I think it's worth reminding people that they are mistaken in believing that satisfying the official 'minimum Zs' requirement does not necessarily achieve the required disconnection times if (as will inevitably be the case in practice) the impedance of thee fault is not 'exactly zero'.

It's perhaps also worth pointing out that this is where RCDs certainly 'come into their own'. Whereas a fault impedance of just a few ohms will totally undermine ADS achieved using OPDs, RCDs will result in very rapid disconnection with fault impedances over 6,000 Ω

Kind Regards, John

 

[ebee]

Quite right there John

 

[ebee]

{L-N} 'short-circuit' protection, since a 'short circuit' is merely an extreme case of 'overload').

No not really John,
at least not as far as we are concerned with things anyway.
They are treated, in our electrical world, as three distinctively seperate things we need to consider A/ Overload, B/ Short Circuit & C/ Earth Fault. Whether different OPDs or the same OPD takes care of one or more of those items, if needed.

We used to pretty much mainly just consider rewireable fuses (or a 6 inch nail ) or sometimessuch a fuse but enclosed in a cartridge.
Then MCBs pretty became used as a replacement for those fuses .

Then RCDs pretty much became back up protection for Earth Faults although being the sole protection for Earth Faults is allowed. To such an extent that some problematic earth fault calculations on some circuit could be corrected by the addition of an RCD, indeed in a TT situation you most often have no choice but to use an RCD or something like it.

 

[JohnW2]

No not really John, ... at least not as far as we are concerned with things anyway. .... They are treated, in our electrical world, as three distinctively seperate things we need to consider A/ Overload, B/ Short Circuit & C/ Earth Fault. Whether different OPDs or the same OPD takes care of one or more of those items, if needed.

That makes sense, but really only if (as in this discussion) one considers situations in which is may be permissible to omit overload protection (but see below about what BS7671 appears to say!) - since, when overload protection is present (and required) there will inevitably also be 'short-circuit protection'.

A fault resulting in an overload can be 'anything', hence covers the entire range of 'effective impedances', extending right down to an effective impedance that is 'infinitely close to zero' - hence, as I suggested, I think it's fair to regard 'short circuits' as the limiting (extreme) case of an 'overload'.

Interestingly (in view of what you say), although it does 'talk about it' (primarily in relation to 'breaking capacity of devices) BS7671 does not seem to have an explicit requirement for short-circuit protection - only 'overload' and 'fault' protection. Table A531 list *** different types of 'overcurrent devices' and for all but one, says that they are suitable for both overload and short circuit protection. The one exception is an "ICB" ("Circuit Breaker, Instantaneous Trip") which I assume is a breaker with a very high In.

An interesting (and perhaps surprising) consequence of this is that, unless I'm missing something, it would seem that if the (BS7671) conditions for omission of overload protection are satisfied, then there is actually no requirement for short-circuit protection. Is that how you read things?

Kind Regards, John

 

[EFLImpudence]

By definition 'overload' can occur in a sound circuit whereas 'short circuit' and 'fault current' are obviously due to faults.

 

[JohnW2]

By definition 'overload' can occur in a sound circuit whereas 'short circuit' and 'fault current' are obviously due to faults.

True, but 'overload' can also result from a 'fault' situation (such as a stalled/jammed motor).

What would you call an excessive current drawn by a piece of equipment due to a fault/condition which, although high, was less than would result from a 'short circuit'??

Kind Regards, John

 

[EFLImpudence]

True, but 'overload' can also result from a 'fault' situation (such as a stalled/jammed motor).

I suppose they all result from a 'fault' of some kind, but not necessarily an electrical fault in the circuit.

What would you call an excessive current drawn by a piece of equipment due to a fault/condition which, although high, was less than would result from a 'short circuit'??

Obviously the lines are very blurred between a fault here or a fault there, but I thought you thought that SS and EF did not necessarily have to be of zero impedance anyway.

I don't know the difference in terms between a fault to earth in a damaged heating element and a damaged cable just outside the appliance.

 

[JohnW2]

I suppose they all result from a 'fault' of some kind, but not necessarily an electrical fault in the circuit.

Sure, but it also could be an 'electrical fault'; (in the connected equipment) - such as a 'short' between two parts of a heating element or two parts of the winding of a motor or transformer.

Obviously the lines are very blurred between a fault here or a fault there, ....

Quite - my view is that it is simply a spectrum of degrees of 'excessive current' - the 'limiting case' being when the 'fault in the equipment approaches zero impedance, such as the current approaches the 'PSCC', dependent only upon the wiring (all the way back to DNO's transformer)

but I thought you thought that SS and EF did not necessarily have to be of zero impedance anyway.

They never will be because, in the real world, 'zero impedance' (and 'infinite impedance') will not exist. That's particularly the case with 'earth faults' since many, particularly those due to water ingress, will be of considerable impedance, such that they would only be 'noticed' by an RCD.

I don't know the difference in terms between a fault to earth in a damaged heating element and a damaged cable just outside the appliance.

I don't really see that there is a need for different terms. As far as the circuit (and the circuit's protective devices) is concerned, it's a 'fault to earth', regardless of its nature or physical location.

After all, you wouldn't want 'different terms' for, say,(a) an overload caused by a washing machine with a jammed motor on a 20A circuit and, (b) an overload caused by three 3 kW heaters being plugged into a 32A socket's circuit, would you?

It's 'nearly Christmas', so don't expect a lot more from me (other than, perhaps, 'Christmas messages'!) in the immediate future

Kind Regards, John

 

[EFLImpudence]

After all, you wouldn't want 'different terms' for, say,(a) an overload caused by a washing machine with a jammed motor on a 20A circuit and, (b) an overload caused by three 3 kW heaters being plugged into a 32A socket's circuit, would you?

Why not? The former being a fault and the latter an overload seems quite logical even if the result is the same.