Boiler Supply Q

[JohnW2]

I still don't see anything obvious about it, in fact I see just the opposite which is why I mentioned 2 different continents.

Fair enough. I obviously can't answer for what you "see", so let me clarify/confirm that what I have been thinking/writing about are EU (and 'similar' countries, and certainly not India.

As you have now expanded the specifics of the cable size, the specific point I (and others) have mentioned is in the UK that is catered for in our regs. Others seem to be saying the cable size seems to be ignored in other countries.

I have seen no suggestions that cable size should be ignored, in any country. What I think has been questioned is how reasonable it is to 'overprotect' the cable supplying item A in case someone connects item B to it using cable smaller than that used to supply item A.

Applying common sense tells me other countries may be putting their installations at risk of overheating.

I don't know enough about regs and practices in other countries, even EU/'similar' ones, to comment on that - but, in context, do you really believe that the likes of stats and CH valves are capable of resulting in significant 'overload' (as opposed to 'fault') currents?

Kind Regards, John

 

[bernardgreen]

Pedant warning

Overload protection is something that protects an overload and allows it to continue.

Protection from overload is something that protects cables and accessories from damage caused by an overload.

 

[SUNRAY]

I suppose I asked for that, by mentioning only the first two of those considerations and not qualifying what I wrote by "when overload protection is required" - since, as you imply, at least in terms of UK regs, there are situations in which such protection may be omitted

Having said that, some people (including Sunray himself, and bernard) are capable of arguing that virtually any sort of of load is capableof causing an overload (under fairly exceptional circumstances)!

Kind Regards, John

As it happens I have a shower heater in the scrap bag (I think it made it in there) which has been tripping its MCB. I never did get round to checking it out.

 

[plugwash]

I suppose I asked for that, by mentioning only the first two of those considerations and not qualifying what I wrote by "when overload protection is required" - since, as you imply, at least in terms of UK regs, there are situations in which such protection may be omitted

Right.

While personally i'm a fan of keeping overload protection as "close" as practical I think it' would be pretty un-likely for the components of a typical central heating system to overload a 0.5mm² flex (even if we use the hugely pessimistic rating of 3A that the IEC assigns it). I looked up a random central heating pump and it's power rating was 34W, if we take the rule of thumb that stall current is often about 10 times higher than running current and pessimistically assume that the pump does not have any overload protection internally then we are still only at 1.5A for a stalled pump.

 

[Deleted member 267285]

we are still only at 1.5A for a stalled pump.

I have been to many pumps that have blown the 3A fuse because water gets in the housing. Also it’s recommended we use 0.75mm squared heat resistant flex.

 

[SUNRAY]

I don't know enough about regs and practices in other countries, even EU/'similar' ones, to comment on that - but, in context, do you really believe that the likes of stats and CH valves are capable of resulting in significant 'overload' (as opposed to 'fault') currents?

Kind Regards, John

Having worked in the controls industry for 20 years I'd say all parts of the controls systems are likely to suffer from this sort of damage and have had to repair all sorts following such problems. So yes to answer they certainly are capable and that is why in the (proffessional/competent as opposed to the likes of W1) controls industry we design in OCPD's.

 

[JohnW2]

While personally i'm a fan of keeping overload protection as "close" as practical I think it' would be pretty un-likely for the components of a typical central heating system to overload a 0.5mm² flex (even if we use the hugely pessimistic rating of 3A that the IEC assigns it). I looked up a random central heating pump and it's power rating was 34W, if we take the rule of thumb that stall current is often about 10 times higher than running current and pessimistically assume that the pump does not have any overload protection internally then we are still only at 1.5A for a stalled pump.

All agreed - and, as I've implied in other posts, I think it's even sillier to start talking/worrying about 'overloads' created by stats or motorised valves etc. !

Kind Regards, John

 

[JohnW2]

Having worked in the controls industry for 20 years I'd say all parts of the controls systems are likely to suffer from this sort of damage and have had to repair all sorts following such problems. So yes to answer they certainly are capable and that is why in the (proffessional/competent as opposed to the likes of W1) controls industry we design in OCPD's.

So what is the mechanism whereby you believe that, say, a thermostat could result in a sustained 'overload' current?

Kind Regards, John

 

[JohnW2]

I have been to many pumps that have blown the 3A fuse because water gets in the housing. Also it’s recommended we use 0.75mm squared heat resistant flex.

I must say that rather surprises me. For a start, there will very commonly be RCD protection, and an RCD would be expected to operate long before a fuse had a chance to blow. However, even in the absence of RCD protection, I have to say that I cannot recall personally ever having experienced an OPD (of any type or rating) operating as a result of current through water. I guess that my experience must be limited!

Kind Regards, John

 

[JohnW2]

Pedant warning ... Overload protection is something that protects an overload and allows it to continue.

Extremely pedantic - and maybe you should tell BS7671 about that, since it uses that phrase extensively to have the meaning that all (but one!) of us expect it to have, rather than what you suggest

Do you believe that the same goes for, say "fault protection" (i,.e. that it means 'to protect faults and allow them to persist')?

Kind Regards, John

 

[SUNRAY]

So what is the mechanism whereby you believe that, say, a thermostat could result in a sustained 'overload' current?

Kind Regards, John

Quite frankly I'd say other than the hysteresis heater going low resistance and it is most unlikely a ¼W resister (or part thereof) will outlive a fuse, there is infinitesimal chance of a stat causing an overload.

I can't predict the same for the unknown device it's controlling though.

However I am able to identify that an arcing thermostat or relay contact (dirty contact/weak spring etc) controlling a 240V contactor will trip a 2A MCB.

The other point to consider is how do we define overload? Any motor rated at over 370W should be overload protected and that we do with an overload sensor attached to the contactor, it is usually set to 110-115% FLC. In a control panel we tend to install such a detector on all motors, even the 34W mentioned earlier.

 

[JohnW2]

Quite frankly I'd say other than the hysteresis heater going low resistance and it is most unlikely a ¼W resister (or part thereof) will outlive a fuse, there is infinitesimal chance of a stat causing an overload.

Quite so - and 'going low resistance' is a pretty rare failure mode for a resistor, anyway

I can't predict the same for the unknown device it's controlling though.

I mentioned stats as a fairly extreme example. However, I would think it incredibly unlikely that any 'parts' (I dare not call them 'components' ) of a domestic CH system could/would overload even a 0.5mm² conductor. As has been said, it's extremely unlikley that even a CH pump could/would.

The other point to consider is how do we define overload?

I think that, in the context of this discussion, we probably all know what we mean. For what it's worth, BS7671 defines an overload current as an 'overcurrent' (defined aa a current in excess of rating of a cable/circuit) which occurs in a circuit which is 'electrically sound'.

Any motor rated at over 370W should be overload protected and that we do with an overload sensor attached to the contactor, it is usually set to 110-115% FLC. In a control panel we tend to install such a detector on all motors, even the 34W mentioned earlier.

Fair enough, but I'm not sure what that has got to do with this discussion.

Kind Regards, John

 

[aesmith]

Do not forget that plug fuses and hence fuses in FCUs only came about because of the invention of the Uk ring final circuit.

Wouldn't they be needed on radials as well, or any other circuit with multiple sockets or connection points?

 

[EFLImpudence]

Wouldn't they be needed on radials as well, or any other circuit with multiple sockets or connection points?

Not because of those reasons.

The fuses (some of them) are required because the ring circuit was designed so that it could be a 30A circuit using existing undersized cable.

Obviously if you have a 30/32A radial the fuse would also be required - but no one else does that either.

 

[aesmith]

Thanks. I was thinking of the scenario where you have several devices on a radial, whether hardwired or connected by plug and socket. To give an example assume 2.5sq.mm fixed cabling, and two or more devices each connected with similar flex of a smaller cross section, let's say 0.75sq.mm. You can't size the MCB to suit 0.75 since there are two or more in parallel.
Whereas with fuses you size the MCB to suit the fixed cabling, and each fuse to suit the actual cabling of the attached device.