Protection against overload current

[studentspark]

Im off work so plenty of time to get myself tied up in knots over (Basic ) electrical theory, so....
I've been thinking about overload, in particular.

433.1.1 and appendix 4

My problem is where overload protection is not required.

And the use of design current, instead of the rated current of the protective device, when applying correction factors. Ci, Cf.....

Im talking domestic, light commercial here. I imagine there is all sorts of systems in place for industrial installations where loss of power could cause serious issue.

So

How is it even possible to omit overload protection?

I presume they are saying a correctly sized cable can handle the design current, and since the load can not be increased, overload protection is not needed , but fault protection is.

But since MCB's provide both its a bit of a mute point.

I read about 2.5mm2 TandE being on a 32A 60898 as an example of not factoring overload, but I would have thought that Ib≤In≤Iz prevents that

Is there such a thing as a fixed load, which can not cause an overload?

Thats not really the case is it? Could a shower /immersion or any fixed load, develop a fault which could cause an overload. If the resistance of the element decreased, the current would increase?


Also..

Equations.. 1-5

The book makes it appear that we only use In when we have a correction factor for the BS 3036 and for grouping. (equations 1 and 2)

But we can also use Cg in equation 5 (Ib)

Is it the case that we only use Ib when we don't have a BS3036 -

It makes it sound like overload protection is only needed when we have a BS3036.

But..

Playing around with cable calculations using Ib and In

Ib often suggests a lower CSA cable, which is only found to be unsuitable when doing the volt drop calculation

Ib often produced a Iz which is right on the edge of acceptability.

In gives you a bit of extra capacity. So is it not best to always use In


When would you use Ib?

Thanks...

 

[EFLImpudence]

433.1.1 and appendix 4
My problem is where overload protection is not required.

Where it cannot happen.

And the use of design current, instead of the rated current of the protective device, when applying correction factors. Ci, Cf.....

Sort of, but you may as well use a lower rated OPD where possible.

Im talking domestic, light commercial here. I imagine there is all sorts of systems in place for industrial installations where loss of power could cause serious issue.

I don't see how that is relevant. If the circuit IS overloaded then the supply should be disconnected.

So
How is it even possible to omit overload protection?
I presume they are saying a correctly sized cable can handle the design current, and since the load can not be increased, overload protection is not needed , but fault protection is.

Correct. 433 and 434.

But since MCB's provide both its a bit of a mute point.

Not really.

I read about 2.5mm2 TandE being on a 32A 60898 as an example of not factoring overload, but I would have thought that Ib≤In≤Iz prevents that

On spurs like that, the fuses in the plugs provide overload protection.
Or one resistive load of less than the CCC of the cable.

Is there such a thing as a fixed load, which can not cause an overload?

Yes, resistive loads like ovens, showers etc.

Thats not really the case is it?

Yes, it is.

Could a shower /immersion or any fixed load, develop a fault which could cause an overload. If the resistance of the element decreased, the current would increase?

No, It's not really possible, is it?

Also..
Equations.. 1-5
The book makes it appear that we only use In when we have a correction factor for the BS 3036 and for grouping. (equations 1 and 2)
But we can also use Cg in equation 5 (Ib)
Is it the case that we only use Ib when we don't have a BS3036 -
It makes it sound like overload protection is only needed when we have a BS3036.

Again, I don't think that is relevant.
If the ring/radial spur cable were down rated to 13.5A (surrounded by thermal insulation) then a double socket would allow overload.

But..
Playing around with cable calculations using Ib and In
Ib often suggests a lower CSA cable, which is only found to be unsuitable when doing the volt drop calculation
Ib often produced a Iz which is right on the edge of acceptability.
In gives you a bit of extra capacity. So is it not best to always use In
When would you use Ib?

...but you would do all that before deciding if the overload protection might be omitted.

 

[EFLImpudence]

Also, you will probably find that most 13A plug fuses are not really necessary as far as the flex is concerned.

Kettles for example cannot cause an overload and the flex will likely be able to handle any fault current which would cause the 32A MCB to trip.

 

[JohnW2]

I presume they are saying a correctly sized cable can handle the design current, and since the load can not be increased, overload protection is not needed , but fault protection is. ... But since MCB's provide both its a bit of a mute point.

As EFLI has said, that's not true.

For example, a 32A or 40A MCB (or maybe even higher rating) might well provide enough fault protection for a (short) circuit wired in 1.5mm² cable, and almost certainly for one wired with 2.5mm² cable, but neither would provide adequate overload protection for those cables.

Kind Regards, John

 

[studentspark]

Thanks EFLI , if I can bother you a bit more.

Could you give me an example of where you would omit over current protection in a domestic setting please.

Then how would you actually go about omitting it.



If the resistance of the element decreased, the current would increase?

No, It's not really possible, is it?

I don't know, the resistance is between the Line and the neutral could that resistance not break down?

I probably look at this wrong, but say an immersion element has a resistance of 19ohms

I see this as 19ohms stopping a short between L and N. a bit like insulation between L and N
If that 'insulation' breaks down, you will get an a decrease in resistance, and an increase in amps

Thanks

 

[JohnW2]

.... I probably look at this wrong, but say an immersion element has a resistance of 19ohms ... I see this as 19ohms stopping a short between L and N. a bit like insulation between L and N ... If that 'insulation' breaks down, you will get an a decrease in resistance, and an increase in amps

What you are overlooking is that the physical nature of a heating element is such that 'L' and 'N' are never physically close (i.e. just separated by 'insulation'). The worst that could happen is that two adjacent bits of the conductor (e.g. adjacent 'turns' of a coiled element) might come into electrical contact (i.e. a 'shorted turn') and, although that would fractionally decrease the overall resistance, it would be a very small effect, and not enough to create anything approaching an 'overload'.

Kind Regards, John

 

[studentspark]

.

 

[JohnW2]

Well yes, fault protection is covered, but they don't provide overload protection
My thought is that they should. so the MCB in sized accordingly less than the CCC of the cable .... sizing the MCB above the CCC of the cable is committing overload protection.

Yes, but we're talking about a situation in which (upstream) overload protection is not required (either due to downstream protection of the fact that (as we have been discussing} the nature of the load is such that it is 'unlikley to create an overload').

In that situation an MCB of adequately low rating to provide the (required) fault protection may not be adequate to provide (not required) overload protection for the cable (as with the examples I recently gave).

Kind Regards, John

 

[studentspark]

Correct. 433 and 434.

As EFLI has said, that's not true.

?

Yes the MCB will offer fault protection. But it can be rated to high to offer overload protection, The MCB rating has to be less than Iz
If you don't have that you don't have overload protection.

But if you don't need overload protection, what does the circuit need to satisfy. Fault protection and design current is less ≤ Iz

 

[JohnW2]

Yes the MCB will offer fault protection. But it can be rated to high to offer overload protection, The MCB rating has to be less than Iz ... If you don't have that you don't have overload protection.

All true -but, if the circuit (cable) is not required to have overload protection, that is obviously fine.

But if you don't need overload protection, what does the circuit need to satisfy. Fault protection and design current is less ≤ Iz

Exactly. If Ib were greater than Iz, then you would obviously have a 'designed in' overload.

'Overload protection' is about protecting cables from currents in excess of the design current. The clue is in the word 'overload'.

Kind Regards, John

 

[studentspark]

Thank you.
I guess my problem is, I don't really understand the design of electrical loads. All I know is that they can pull a current, and some how that current is limited.
And I have it deep in my thinking that any electrical circuit can overload, and cause disaster.

I blame watching the Towering Inferno as a kid...

 

[bernardgreen]

And I have it deep in my thinking that any electrical circuit can overload, and cause disaster.

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.

 

[ericmark]

I have seen red 13 amp plugs clearly marked NHS which do not have fuses, but that is the only time I have seen it were there is no overload protection, and they are only fitted to machines where failure would mean loss of life.

Calculations for volt drop use design current, so for example the standard for a ring final is to consider 20 amp drawn at centre of ring, and 12 amp even spaced, so for volt drop we use 26 amp not 32 amp but it is still 32 amp for over load protection.

 

[JohnW2]

Thank you. I guess my problem is, I don't really understand the design of electrical loads. All I know is that they can pull a current, and some how that current is limited.

It's only in relation to hard-wired loads (like immersions) and/or 'dedicated' circuits (e.g. for cookers) that a designer has actual 'control' over, or definite knowledge about, the load.

With a sockets circuit, a designer can but use their judgement to guesstimate what the load is likely to be. If people plugged 3kW loads into every socket outlet on a circuit, the total load could be incredibly high.

And I have it deep in my thinking that any electrical circuit can overload, and cause disaster.

Little in this world is literally 'impossible'. However, as we've been discussing, it's extremely unlikley that dedicated circuits supplying essentially resistive loads (usually 'things which produce heat') will ever suffer an overload (although they may, of course, suffer 'faults').

Kind Regards, John

 

[JohnW2]

I have seen red 13 amp plugs clearly marked NHS which do not have fuses, but that is the only time I have seen it were there is no overload protection, and they are only fitted to machines where failure would mean loss of life.

That's true, although it has always seemed to me to be more than a little optimistic to believe that will have much practical value - since if a machine has developed a fault which causes it to blow a 13A fuse, it is extremely unlikely that the machine will still be functioning - satisfactorily or at all.

However, the circuits supplying sockets into which those reg plugs are plugged will have upstream overload protection.

Kind Regards, John