Protection against overload current

[JohnW2]

Any electrical item can go faulty and and as a the result of that fault draw more than it's design current, some items have fault protection in the form of a fuse internal to the item.

As we've been discussing, there are some (primarily resistive) loads which are incredibly unlikley to develop in an 'overload' current, although any can, of course, give rise to a 'fault' current (i.e. due to some sort of 'short-circuit'.

The 'nightmare' faults (the the everyday, not technical, sense) are those which are of low, but not 'negligible' impedance, since there's really no way we can, at least at present, realistically protect against them with over-current devices. One would need devices that had much lower 'magnetic trip thresholds' than our current ones - but that would then probably result in a lot of 'nuisance' tripping (since the thermal part of present-day OPDs is pretty tolerant to relatively short-term 'overload' currents).

Kind Regards, John

 

[securespark]

AFAIR, the appliance faults I have visited that were protected by MCBs have all resulted in the MCBs tripping before the 1362 blew.

 

[JohnW2]

AFAIR, the appliance faults I have visited that were protected by MCBs have all resulted in the MCBs tripping before the 1362 blew.

That seems to be the usual experience, and it seems to make total sense - it has to take a finite time for a fuse to 'heat up', but an MCB will clear the fault in a small number of milliseconds.

Similarly (and I presume for the same reason), it's very rare for a cutout fuse to blow when an MCB trips, even though the fault currents will be plenty large enough that they would have blown the fuse if allowed to flow for any appreciable amount of time.

Having said that, I'm sure I'm not the only person who has seen a BS1362 fuse blow (as a result of a fault) but with the corresponding MCB not operating - so there is an element of 'pot luck' in it (perhaps related to the 'age' of the fuse - maybe 'aged' fuses are more trigger-happy?).

Kind Regards, John

 

[securespark]

Having spoken to many a Norweb spark over the years, they tell me that cutout fuses often fail with age, especially the very old 3036s.

 

[JohnW2]

Having spoken to many a Norweb spark over the years, they tell me that cutout fuses often fail with age, especially the very old 3036s.

That makes sense, although I think it is probably still very rare, even with aged fuses.

In my entire life to date (a good few decades!), the only occasion on which I have personally been aware of a cutout fuse blowing was when (as I described and illustrated here), an RCD in my daughter's CU literally blew up when she pressed its test button.

Kind Regards, John

 

[EFLImpudence]

Surely you both did not mean 3036s?

 

[JohnW2]

Surely you both did not mean 3036s?

I made no comment about the type of the cutout fuses. Presumably, despite what secure wrote, they never were 3036s (I confess that I did not 'take in' what he'd written - otherwise I, too, would have questioned it!).

Kind Regards, Joh

 

[securespark]

I can only go off what these Norweb sparks told me.

From personal experience I have known the old cast iron cut-outs to have 30A fusewire in. I guess after a few decades with an ever-increasing load, that could occur.

 

[SimonH2]

Bear in mind that many heater elements CAN fail in a manner that draws a higher current. Take your common 3kW immersion heater, and lets say it develops a fault L-E in the middle of the element. It'll now draw 4x it's nominal current - so around 52A. That could take a while to trip (say) a B20 MCB. Obviously the fault could be nearer the neutral end of the element - in which case the overload will be somewhat less, potentially to the point where the MCB or fuse will lever trip.
Of course, these days it's highly unlikely that the circuit won't have RC protection which would disconnect the supply - but that's not relevant in the context of the original question.

 

[EFLImpudence]

Bear in mind that many heater elements CAN fail in a manner that draws a higher current. Take your common 3kW immersion heater, and lets say it develops a fault L-E in the middle of the element. It'll now draw 4x it's nominal current - so around 52A.

No, it won't.

 

[bsr]

Could someone answer the question in the OPs second post? Can you give some examples of domestic situations where this would make a difference to the circuit?

For example does this rule mean you could fit a 45A or 50A MCB to an 8kw electric shower circuit on 6mm or a 13A immersion on 2.5mm since the MCB would still provide fault protection through the magnetic trip?

OP I know that at work some of the safety-critical medium voltage stuff (3-11kV) runs on unfused supplies and instead runs IT earthing and has contactors linked to fault detection circuits. I believe this is done because the safety case says the risk of life from a nuisance trip could be higher than that from an overload.

 

[JohnW2]

Could someone answer the question in the OPs second post? Can you give some examples of domestic situations where this would make a difference to the circuit? .... For example does this rule mean you could fit a 45A or 50A MCB to an 8kw electric shower circuit on 6mm or a 13A immersion on 2.5mm since the MCB would still provide fault protection through the magnetic trip?

One would have to undertake the necessary calculations for the circuit to determine whether or not the MCB did provide adequate fault protection, but it's very likely that it would (certainly in the former, and quite probably also the latter, situation) - and then if fault protection were adequate, then, yes, that would be OK.

Kind Regards, John

 

[JohnW2]

Bear in mind that many heater elements CAN fail in a manner that draws a higher current. Take your common 3kW immersion heater, and lets say it develops a fault L-E in the middle of the element. It'll now draw 4x it's nominal current - so around 52A. ....

Why "4x" (rather than 2x)?

We've been through this before. What you say is obviously not impossible (although, as you say, is pretty moot these days given the ubiquity of RC protection) but I don't think that one can really say that it is "likely" to happen - and it "being unlikely" is all that the regulations require in order for it be permissible for overload protection to be omitted.

Kind Regards, John

 

[SimonH2]

Why "4x" (rather than 2x)?

Ah, brain fade Must have had in mind the scaling factor changing two resistors in series to parallel. But the principle remains - things can fail in a manner that significantly increases current.

Thinking a bit more ... Motors and transformers can have inter winding shorts - increasing current but not (as with most heaters) needing an earth fault.

 

[JohnW2]

Ah, brain fade Must have had in mind the scaling factor changing two resistors in series to parallel. But the principle remains - things can fail in a manner that significantly increases current.

They can (which probably means that they very occasionally do). However, as I said, in the case of heating elements, I would suspect it is very rare - and certainly not common enough to constitute "being likely to result in an overload current" - which is all that matters to the regs in relation to the omission of overload protection.

To put it another way. Whoever wrote the regulation must have believed that there were some situations in which the 'nature of the load' was such that an overload current was 'unlikley' - and I can't think of any sort of load for which it would be less likely than an essentially purely resistive heating element.

Thinking a bit more ... Motors and transformers can have inter winding shorts - increasing current but not (as with most heaters) needing an earth fault.

Very much so - which is why I always say, for example, that anything involving a 'significant' motor cannot be regarded as 'unlikely to result in an overload current'.

However, there's a far more important reason for that than 'shorted turns' - a (mechanically) stalled/jammed motor can result in large overload currents, even without anything being electrically wrong (like a 'short').

Kind Regards, John