any ideas?

[Patto]

Had a fault on some canopy lights (250w metal halide) the fault was a snapped pyro at a joint box going out to the lights, the electrician decided that rather than get up to the lights to identify the cores he would just take a guess now the lights came on but the pyro feeding them got extremely hot even melting the outer sheath so he tried them the other way and its now fine so obviously reversed polarity. . My question is what would cause that??? Theres a choke ignitor and a capacitor aswell as the lamp, he had no explanation for it, and as im still a student im interested as to why....

 

[MDJ]

Had a fault on some canopy lights (250w metal halide) the fault was a snapped pyro at a joint box going out to the lights, the electrician decided that rather than get up to the lights to identify the cores he would just take a guess now the lights came on but the pyro feeding them got extremely hot even melting the outer sheath so he tried them the other way and its now fine so obviously reversed polarity. . My question is what would cause that??? Theres a choke ignitor and a capacitor aswell as the lamp, he had no explanation for it, and as im still a student im interested as to why....

Gasp, a pyro melting is something I have never come across in 33 years in the trade

 

[Patto]

Not the pyro just the orange outer sheath

 

[tbs1983]

The only thing i can think of is that it effectively went through the lamp in reverse because of your polarity error missing out the ballast and capacitor which are there to restrict the flow, therefore it was effectively like putting a metal halide lamp into a ceiling rose (if that makes sense). Perhaps the large start up current was enough to heat cable enough and melt, although why fuse didn't blow/trip first I don't know.

n.b. anyone please feel free to tell me if you think I'm talking absolute tripe, its just a theory but im not 100%

 

[Darkwood]

Low insulation resistance - damp in the repaired cable i would use the term 'Electrician' loosely a competent Electrician doesn't play guessing games with polarity etc if he'd hae tested the repaired circuit prior to energising he would have identified any issues and corrected them.

 

[Dizzy_Maskell]

Low insulation resistance - damp in the repaired cable i would use the term 'Electrician' loosely a competent Electrician doesn't play guessing games with polarity etc if he'd hae tested the repaired circuit prior to energising he would have identified any issues and corrected them.

That depends if it was late on a Friday afternoon he probably just wanted get the light on and jump back on his horse yehaaaaaaaaaa

 

[Lucien Nunes]

The overheating cable would not have been caused by reverse polarity of the light fixture. There could be all sorts of other faults and like Darkwood says this sounds like a very cavalier attitude but an AC circuit is an AC circuit and most will work correctly regardless of polarity. Getting the polarity right is mainly about ensuring that switches, OCP etc are in the line not the neutral.

 

[Patto]

That's what he said polarity wouldnt make much difference, the wrong attitude to me, I just couldnt understand why it it hot extremely hot but when reversing them it worked no problems.

 

[DPG]

Poor connection when he connected them up the first time? Daz

 

[nickblake]

agree with DPG poor connection , could possibly be a low insulation resistance but would expect the protective device to operate , unless of course it was overated

 

[itselectric]

That's what he said polarity wouldnt make much difference, the wrong attitude to me, I just couldnt understand why it it hot extremely hot but when reversing them it worked no problems.

How about a high resistance to earth fault on one core after new termination. After reading op's post I would guess the " electrician" didn't test and just coupled up. Just unlucky (if you can call it that) that he connected the faulty core to the live. Could get hot, then swap it round and the fault core becomes the neutral and won't get hot.

 

[Patto]

This is the scenario 1 x 4L 1.5 going out to a joint box then 2 x 2L1.5 going up to the lights one cable has 3x 250w halide canopy lights the other has 2 canopy lights, he connected the first up and the cable got hot so swapped the cores and it was ok but the same happend with the other cable....

 

[Lucien Nunes]

To get hot enough to melt PVC, you need hundreds of watts of dissipation in a short length. That means an 'insulation resistance' in the order of a few hundred ohms, which is unlikely just to be moisture, usually it's tens or hundreds of kilohms or a complete short circuit, in which case the seal will blow out or the breaker will trip. I suspect the moisture has caused the cable to arc inside in which case the insulation will be permanently damaged. It needs to be tested properly, just because it is working now does not mean all is well.

 

[Patto]

The seal did lift and the compound ran out a little, im not saying all is well because it works not in the slightest, im not qualified I have just started my level 3 after completing level 2 with distinction, im very interested in the science behind it hence wanting to know why this would happen.. he seemed happy enough to leave it, he did test the cables but presumably got false readings with the lamps and control gear connected.

 

[Lucien Nunes]

To be certain of a pyro repair you need to test the insulation from every core to every other core and to the sheath and you should not be satisfied with anything less than overrange on any ordinary tester. In a perfect world you would test again after 24 hours to ensure that any seals you have fitted are not leaking. Obviously you can't do a complete test with the lamps connected, so I would say the tests were incomplete and missed the fault that was still present. Surely if you had a failed seal it would all have had to be disconnected to remake it, in which case it would have been possible to test properly?