Ceiling light fitting help

[kendor]

then why the hell is single red and earth made?

where did you find this info?

It would be a case of wrong application for the job, addendum: If a back to back switch arrangement has only the live passing between switches as suggested then that would mean two runs either side of the wall running up to lights, this is bad wiring practice you should use feeds and returns between the switches and then run up to the corresponding lights making sure at no point that there are single core runs, use of multicores may have to be used in order for this not to happen.
The cable in question is a modified T&E(Black cut short)
I didn't find this information, It was drummed into me during my training as an electrician years ago.

I have seen the damage that eddys can do in practice.

Don't they teach you about the hazards of running single cables anymore?

 

[kendor]

Dug up some old college notes:
"Eddy-currents are created by the magnetic field found around a cable when an electrical voltage/current is present. This magnetic field builds up and collapses as the voltage varies. When the field collapses it induces an opposing voltage back into the cable(back emf). This opposing voltage causes eddy-currents in the conductor these are currents that add to the designed current of the circuit and therefore will not have been taken into consideration when calculating cable size, a neutral or return path cable should be run in the conduit or trunking, the opposing magnetic field will cancel out any eddy current effects from being produced."
Although in the above it states conduit or trunking the theory is the same as the effects congregate around metalwork or are exaggerated by surrounding metalwork, metal switch boxes come to mind here.

 

[securespark]

SPEAK UP, KEN!!

 

[kendor]

SPEAK UP, KEN!!

???

 

[kendor]

As an afterthought i've elaborated on one of my previous posts above in an addendum.

 

[ban-all-sheds]

Dug up some old college notes:
"Eddy-currents are created by the magnetic field found around a cable when an electrical voltage/current is present. This magnetic field builds up and collapses as the voltage varies. When the field collapses it induces an opposing voltage back into the cable(back emf). This opposing voltage causes eddy-currents in the conductor these are currents that add to the designed current of the circuit and therefore will not have been taken into consideration when calculating cable size, a neutral or return path cable should be run in the conduit or trunking, the opposing magnetic field will cancel out any eddy current effects from being produced."
Although in the above it states conduit or trunking the theory is the same as the effects congregate around metalwork or are exaggerated by surrounding metalwork, metal switch boxes come to mind here.

OOI - what sort of magnitude are these eddy currents? Your typical switch cable in a lighting circuit is going to have at most an amp or so flowing in cable rated at at least 10A, possibly more than 15A. Do you really get induced currents which are an order of magnitude greater than the inducing one?

You say you've seen the damage that eddy currents can do - what were the circumstances? Was it a domestic lighting circuit?

 

[kendor]

OOI - what sort of magnitude are these eddy currents? Your typical switch cable in a lighting circuit is going to have at most an amp or so flowing in cable rated at at least 10A, possibly more than 15A. Do you really get induced currents which are an order of magnitude greater than the inducing one?

You say you've seen the damage that eddy currents can do - what were the circumstances? Was it a domestic lighting circuit?

What i find strange is that individuals on here will quote the regs and blast people for diverting from them but when it comes to basic electrical principles then anything seems to go? Just because something hasn't failed yet doesn't mean it won't in the future if the circumstances are right.
I'm not directing this to you personally but in general to the replies received, it seems that if the regs don't mention it then it is ok to do something even if it may be bad practice.

Worst example i've seen, although not a lighting circuit in itself, were 4x25mm single SWA's(3P+N) feeding a dist board for lighting the cables ran together until they were terminated by the use of Plastic glands (Yes Plastic) whereby they were double set wide enough to allow for the glands into a steel trunking, the neutral was badly burnt around the point where it passed through the trunking, a specialist firm was called in and confirmed that the damage was due to eddy currents and as rewiring was not an option for the site owners modification was carried out to the trunking to allow the damaged part of the cable to be removed and the firm modified the trunking by cutting small slots at angles to the glands seperating them from allowing the magnetic effects to reoccur, this seemed to do the trick. I know the reultant job may not have been up to specs as they still kept the singles and the plastic glands(there was a seperate earth) etc but you can only advise clients that you are unhappy with something and it's up to them.
regarding the magnitude I may have misled you by stating that the current could exceed the design current I stated this as i remembered from my college days mention of eddy currents as small swirls of current that could be many magnitudes the current flow through the conductor but as these swirls are localised then we are not talking current flow through the entire conductor it is this lost energy localised( the amount of eddys generated is a science in itself, As eddy current calculations were not in the courses remit I was never taught them,so cannot quote figures) that generates heat and it is this localised heat which is the danger.

 

[Stod]

4x25mm single SWA's(3P+N) feeding a dist board for lighting the cables ran together until they were terminated by the use of Plastic glands (Yes Plastic) whereby they were double set wide enough to allow for the glands into a steel trunking, the neutral was badly burnt

How come the neutral got burnt and not the other three phases? If they were balanced, the neutral current would be zero. It's maximum would be no more than the phase with the heaviest loading.

I have seen slotted entyies for single cores, but only on relatively high current circuits. I can't believe it could be a problem for lighting circuits.

Single + E was used a lot up to 1970 or so. I have always regarded the current practice to be to limit EMR.

Easy way to test kendors theory though!

 

[ban-all-sheds]

What i find strange is that individuals on here will quote the regs and blast people for diverting from them but when it comes to basic electrical principles then anything seems to go? Just because something hasn't failed yet doesn't mean it won't in the future if the circumstances are right..

Well - I don't think that "anything goes", but "basic electrical principles" don't need to be applied to every last detail in every situation. For example, with the 1-10mm cables and up to 50A currents encountered in domestic wiring it is not worth considering the reactive component of conductor impedance, as it's so small. Pure resistance is all that you need.

All I did was to wonder if eddy currents could possibly be significant in a lighting circuit carrying an amp or so (probably less), and what you did was to quote me an example of a 3-phase circuit wired in 25mm single core SWA - not very close to the sort of environment that concerns me.

The effects and the problem that you saw seem to be very similar to those described and illustrated here: http://www.tlc-direct.co.uk/Book/4.2.4.htm , where they make the point that single cores surrounded by ferrous material will generate heat in that material, and they actually say that single-core armoured cables should not be used.

I don't doubt the presence of eddy currents in single-core lighting circuits, but I am still unconvinced that they could be significant.

 

[kendor]

How come the neutral got burnt and not the other three phases? If they were balanced, the neutral current would be zero. It's maximum would be no more than the phase with the heaviest loading.

I have seen slotted entyies for single cores, but only on relatively high current circuits. I can't believe it could be a problem for lighting circuits.

Single + E was used a lot up to 1970 or so. I have always regarded the current practice to be to limit EMR.

Easy way to test kendors theory though!

It wasn't a balance problem, as i stated the specialist firm called in confirmed the damage was caused by eddy's it was localised to where the cable entered the trunking, the fact that only the neutral got burnt is anyone's guess?

 

[kendor]

I don't doubt the presence of eddy currents in single-core lighting circuits, but I am still unconvinced that they could be significant.

Why would they teach you that it is bad practice then? It was drummed into us not to wire single runs anywhere as it can cause eddy's so why would they tell you this if it wasn't a problem?

 

[ban-all-sheds]

Why would they teach you that it is bad practice then? It was drummed into us not to wire single runs anywhere as it can cause eddy's so why would they tell you this if it wasn't a problem?

It's quite possible that they taught you that because you might be involved in installations with great thick cables carrying 100's of amps, and rather than have you be concerned about the point at which eddy currents would become a problem, particularly as the whole subject is so indeterminate, they taught you that simple and unvarying practice.

I'm not really arguing with you, just trying to get a handle on it - I can see why a "do not ever do it" rule makes practical sense if you want one rule to cover all LV installations. But even allowing for anomalous behaviour, from what I know of the physics, I'm not sure that if I am installing a switched live rated at 10-15A, carrying a current of less than 10% of that, that in practice I have to be concerned about eddy currents.

 

[kendor]

It's quite possible that they taught you that because you might be involved in installations with great thick cables carrying 100's of amps, and rather than have you be concerned about the point at which eddy currents would become a problem, particularly as the whole subject is so indeterminate, they taught you that simple and unvarying practice.

I'm not really arguing with you, just trying to get a handle on it - I can see why a "do not ever do it" rule makes practical sense if you want one rule to cover all LV installations. But even allowing for anomalous behaviour, from what I know of the physics, I'm not sure that if I am installing a switched live rated at 10-15A, carrying a current of less than 10% of that, that in practice I have to be concerned about eddy currents.

Funnily enough it was lighting circuits that they were using as an example when they taught me. I'm just really surprised that no-one has heard or been told about this. maybe they can't fit in all experiences and knowledge into the condensed courses they have these days?