So how much power does an oil filled radiator actually use.

[T i m]

I don't think you can be sure exactly where the voltage drop is coming from though can (even) you?

I'll look forward to that moment for you. ;-)

Cheers, T i m

[whisky-dave]

No but againn I donlt care. We have those 3 pin sockets in the lab that supply power that are just like those in everyones house, hwo often to yuo worry whether the volatge at that point is 200V or 240V ? How often to you measure it to check, before vacuuming do you check the voltage before or during vacuuming if not why not ?

[T i m]

I'm guessing you do care or this thread wouldn't be as long as it is?

When I bought a heater that was supposed to be 2kW and I got far less than I expected?

I would if the vacuum didn't perform as expected and FWIW, I have a voltage monitor plugged in all the time.

Next!

Cheers, T i m

[John Rumm]

Hmmm Dunning Kruger at work perhaps...

If the wiring is underspecified, that would suggest that close to full load it is running at over them maximum continuous conductor temperature rating of the cable. So expect shortened cable life and elevated fire risk.

If the voltage sags too far, then you get lower PSCC so can expect slower operation of protective devices under fault conditions, so increase electrocution risk, and catastrophic cable failure risk.

[whisky-dave]

so why did we pay £30K for higher capacity electrics ?

Over the life of teh cable we ran it for about 2 and a bit hours after teh

32A MCB tripped we removed 3 of teh 5 heaters and put 2 on another 32 amp M CB and the final heater on another phase our 'CU" has a 'cut-out rated at 100 AMPS. I di not think the life of the cable will be significantly shortened becaus e we have 5x 2Kw heaters that spend most of there time at 700W .

But not in under 3 hours.

[Andy Burns]

The 3 hours at 40+ Amps is the "slow" part of the MCB curve, in the event of a short circuit you want sufficient current to ensure it trips within the "fast" part of the curve ...

[John Rumm]

I am assuming you did not have a rewire done just prior to running this experiment?

If this experiment *has* demonstrated that the installation was sub standard, then it has likely been that way for some time.

It was not the 3 hours that would bother me. Especially if anyone is considering running with this daft idea for any extended period.

[John Rumm]

Indeed...

You need 160A to make sure a B type MCB will trip in the magnetic part of its response curve and that implies a maximum loop impedance at the point of the fault of 230 / 160 = 1.44 ohms [1]

Even if the circuit is in spec and meets that requirement at all sockets, running at only 202 volt gives a reduced potential worst case PSSC of 202 / 1.44 = 140A, which could leave you on the thermal portion of the response curve and *20 seconds* away from disconnection. (and that ignores the effect of elevated conductor temperature on the loop impedance)

[1] I am assuming that the wiring was done before 17th edition amendment 3 and the 5% reduction in Cmin.

[whisky-dave]

what makes you think it's 40 amps + ?

[Andy Burns]

I thought that's where we started weeks ago, that you expected to be able to trip a 32A breaker with the heaters, and I gave you the 2.7 hours figure?

As John has calculated, if the supply is so "soft" that it drops to

202V, it might only be able to supply 140A, so a short circuit could last long enough to cause serious damage ...

[whisky-dave]

what makes yuo think it;s 40 amps will you answer this or not.

only 140 amps where or how did you get that figure ?

[whisky-dave]

So why did it trip out ?

140A I don't think we were drawing anywhere near that. Is that the sort of current you expect from 5 2KW heaters running on 202V and a soldering re-work station of about 160W max. ?

I'm not sure that is relevant to this.

[whisky-dave]

It's not me that's making the thread long.

Mine was a simple question regarding what power would yuo expect a 2KW heater to use. I wanslt expecting pin point accuracy when I knew the jeaters specs was 2KW 220-240V there;s a 10% change without the power altering.

Not sure about you but if I put the heater on a 220V supply I'd expect it to consumer a bit less power than it would on a 240V supply. Taking it up to 260V would be beyond it's quoted spec. and I;d assume that the power would increase until a cut-out tripped out protecting the elemants and the heater in gerneral.

Did yuo take it back or report it as faulty ?

[Andy Burns]

I do not think your heaters are taking 40A. We were discussing why the MCB didn't trip, because a 32A breaker can supply more than 40A for hours without tripping.

You just glibly replied to Johns message where he showed you ... he wasn't saying your heaters are taking 140A, rather that in the even of a short circuit (where ideally you'd want many hundreds of amps available to ensure the MCB trips in under half a second) your 202V supply could only supply 140A which would take it 20 seconds to trip.

140A at 202V is 22kW heating up your cabling, your lab should be toasty until the fire service arrive.

[Andy Burns]

Sorry 28kW.

[John Rumm]

It didn't. At least not in the magnetic part of the response. Neither would you expect it to in the absence of a *fault current*.

Go nail through a cable and measure it again!

No, you are confusing overload current with fault current.

A fault current is what you will see when something bad happens in a big way; say a cable or flex is damaged and a short circuit between live conductors or live and earth occurs.

In these situations the current that flows will be limited only by the round trip resistance of the circuit's cables. Faults of this nature will result in rapid adiabatic heating of the cable's conductors. Unless the current is interrupted rapidly then cable damage will occur, and this could include it melting, charring, or bursting into flames.

MCBs are designed with a separate trip mechanism. One of which is intended to handle this situation *quickly* (typically under 0.1 secs) using a solenoid to trip the mechanism (this is independent of the bi metal strip that provides the normal *overload current* protection).

For this to work, the MCB needs to see enough fault current. For a type B device this could be up to 5x its nominal rating - so 160A for a B32 device.

It is, and I will explain why.

Real world results[1] demonstrated that the maximum loop impedances allowed in the standard circuit designs presented in BS7671 were slightly too high, since when circuits designed to these specs were operated in combination with supply voltages toward the bottom of the allowable supply range, you could not always rely on the protective devices operating quickly enough to clear faults.

Hence amendment 3 of the 17th edition applied a 5% reduction in the maximum allowable loop impedances. e.g. that specified for a B32 device was reduced from 1.44 ohms to 1.37 ohms.

That means that a circuit installed today that meets the new requirements will be more likely to clear faults even when the supply voltage is as low as 216V.

Since your circuits are likely to pre-date this change, they will at best only be compliant with the previously specified maximum impedance, and hence you will see a progressively increasing risk that a fault will not clear promptly at supply voltages under 230V.

Note: for the avoidance of doubt this is an issue that will be of most concern on longer circuits - i.e. those approaching the maximum permitted cable length. It will also be dependent on the MCB in question, since the permitted range of fault currents detected go from

3x to 5x In for a type B device so will be a problem only for less sensitive examples.

[1] Documented in: CENELEC: "Technical Report PD CLC/TR 50480:2011 Determination of cross sectional area of conductors and selection of protective devices"

[John Rumm]

Its not relevant anyway since you have seized on the wrong bit. The takeaway message was in the second bit of Andy's sentance: "...is the 'slow' part of the MCB curve, in the event of a short circuit you want sufficient current to ensure it trips within the 'fast' part of the curve"

[whisky-dave]

What makes you think it could only supply 140A ?. Isn't that the case with all 32A MCB anywhere in the country so if that is a fault of the MCBs maybe they need redesigning. Suppose we were drawing 32 amps (4 heaters) what would be the calculations then ?

What makes you think that

but it wouldn't be 140A at 202V would it. If we can only get 40A at 202V what makes yuo think we could get 140A at 202V surely as the current increased the voltage between L-N would drop. It droped from ~215V at 0A to ~202V at 40A, I'm pretty sure that once it got to 50 Amps the voltage would drop below 200V.

[whisky-dave]

So what happened ? a student was soldering and he said my soldering statio n has stopped working.... this was on the same trip as the heaters who's LE Ds had also just gone out. When we looked in the riser cupboard where the C U is the MCB has tripped or cut out or whatever you prefer to call it. The heaters (3 of them) were removed the other were switchd off, and the MBC sw itch put to the ON position and those things that went off came back on aga in so how did that happen ?

Measure what ?

2V and a soldering re-work station of about 160W max. ?

I'm not as current is current there is NO difference, it's just electrons a nd charge.

Do you think the heaters or the soldering iron produced his fault current. ?

Yes and will this happen at 32A 40 A 50 A and how long will it take ?

what situation ?

So which tripped out ?

So are you saying that the professionals or the company employed to install these MCBs installed cable that wasn't up to carrying the 32A 40A or 140A or 160A you say the MCB is designed for ?

The inspection panel has a date of 16th june 2013.

[Andy Burns]

The MCB doesn't really limit how much current you can draw, it just cuts off if you happen to take too much for too long, it's the length and cross-section of your cables that limit what you really can draw

stick a 6" nail through the submain coming into your lab and find out.

bzzzt, you've got to be trolling ...