Street lighting fault

[normanton]

I'm going round in circles with this fault. We have 12 street lighting lamp posts at our factory, 150W son lamps in each. They were on a 20A MCB and yesterday started tripping. They are basically wired from a 20A MCB (2.5mm), through a relay, out of the relay in 6mm armoured and then outside, where somewhere along the line they are jointed to MICC cables before going to the lamp posts . With me so far? One out of monkeys decided to simply upgrade the MCB to a 32A, which actually held in, even though the cable and MCB for red hot. Obviously this isn't the answer so I've put the 20A MCB back in temporarily. Now, my first guess was that there was a loose connection somewhere which was causing the MCB to get hot, but my understanding is that resistance causes a voltage drop and when I checked the voltage at the posts, it's still hovering around 240 (granted it could have been higher before the fault occurred). Secondly, I've just put the clamp on and it's pulling around 36A, circuit SHOULD be around 7.5A (12 X 150W). But again I've always been under the impression that a high resistance creates heat because there's a large voltage drop across it and the current would more than likely decrease in fact. Anyway, I'm at the end of my tether with this one. Any help would be much appreciated.
Any HELP would be. Could the usual patronisers please not comment, I'm looking for advice NOT a Lecture!

 

[Kamikaze]

Ok, step No 1: Disconnect the 6mm wires (live and neutral) from the load side of the relay. Get your insulation resistance tester and get insulation resistance test readings for (live to earth) and (neutral to earth). Report back with your findings and you will be ready for step No 2.:43:

 

[normanton]

Ok, step No 1: Disconnect the 6mm wires (live and neutral) from the load side of the relay. Get your insulation resistance tester and get insulation resistance test readings for (live to earth) and (neutral to earth). Report back with your findings and you will be ready for step No 2.:43:

Trying to get hold of a IR tester as we speak
keep you posted.. Thanks

 

[Pete999]

Ok, step No 1: Disconnect the 6mm wires (live and neutral) from the load side of the relay. Get your insulation resistance tester and get insulation resistance test readings for (live to earth) and (neutral to earth). Report back with your findings and you will be ready for step No 2.:43:

With Kami on this one

 

[Pete999]

Your description? is the joint between the SWA and MICC t each lamp post, or has someone wired the entire circuit via one transition between SWA nd MICC?

 

[normanton]

Your description? is the joint between the SWA and MICC t each lamp post, or has someone wired the entire circuit via one transition between SWA nd MICC?

Whole circuit mate. Basically the DB was moved a few years back and a new swa was run in from the DB to the first lamppost or first underground joint. All the existing MICC is still in use.

 

[Pete999]

Whole circuit mate. Basically the DB was moved a few years back and a new swa was run in from the DB to the first lamppost or first underground joint. All the existing MICC is still in use.

Disconnect the SWA / MICC joint and see which bit of cable the faults is on

 

[Hellmooth]

Don't think he knows where the joint is, would be an idea to look into into though!

 

[normanton]

Ok, step No 1: Disconnect the 6mm wires (live and neutral) from the load side of the relay. Get your insulation resistance tester and get insulation resistance test readings for (live to earth) and (neutral to earth). Report back with your findings and you will be ready for step No 2.:43:

IR readings as follows:

L-N 0 meg ohms (obviously as lamps are all still in circuit)
L-E 50 meg ohms
N-E 45 meg ohms

*scratches head.. *

 

[Kamikaze]

IR readings as follows:

L-N 0 meg ohms (obviously as lamps are all still in circuit)
L-E 50 meg ohms
N-E 45 meg ohms

*scratches head.. *

Ok, step No 2: Leave the 6mm switch wire and neutral at the relay disconnected. Put them into connectors or make sure they are not touching each other or are touching the earth. Go to each lamppost in turn, disconnect the flex from the fused cut out (assuming there is one) going up to the light. Test the insulation resistances of the flexes. Any flexes with low live or neutral to earth insulation resistance readings leave disconnected, all others can be reconnected. While you are working your way along the lampposts in tern, also test the insulation resistances of the supply cables. Do not worry if you have disconnected a flex and the supply cable still has not cleared. Just keep working along the lampposts in a logical order. Do not skip any or make any other assumptions. Report back with your findings and you will be ready for step No3.

 

[normanton]

Ok, step No 2: Leave the 6mm switch wire and neutral at the relay disconnected. Put them into connectors or make sure they are not touching each other or are touching the earth. Go to each lamppost in turn, disconnect the flex from the fused cut out (assuming there is one) going up to the light. Test the insulation resistances of the flexes. Any flexes with low live or neutral to earth insulation resistance readings leave disconnected, all others can be reconnected. While you are working your way along the lampposts in tern, also test the insulation resistances of the supply cables. Do not worry if you have disconnected a flex and the supply cable still has not cleared. Just keep working along the lampposts in a logical order. Do not skip any or make any other assumptions. Report back with your findings and you will be ready for step No3.

Great stuff, will do. Only question I have is, if there was a fault on the head, wouldn't the fuse in the lamp post blow before the MCB? Providing there is discrimination of course. There are 6/10A fuses in the lamp posts by the way.

 

[Kamikaze]

Not necessarily because the problem might not be on one of the heads and could be the supply cable. If the problem is the supply cable, it will most likely be that one of the MICC pots has kicked the bucket. But you can’t make that assumption just yet, since you haven’t established whether any of the heads are down.

 

[Pete999]

Not necessarily because the problem might not be on one of the heads and could be the supply cable. If the problem is the supply cable, it will most likely be that one of the MICC pots has kicked the bucket. But you can’t make that assumption just yet, since you haven’t established whether any of the heads are down.

Doin a good job here Kam

 

[Kamikaze]

Doin a good job here Kam

I’m in a very amenable mood today. But don’t worry it wont last…lol.
This thread has reminded me of a recent saga. The clowns in the office where I work send another clown to a school because the outside lights were tripping. Days he spend there! Did I mention it was DAYS!:13: He came away without having figured out what was wrong. Instead of following a logical order and eliminating what was ok from what was not ok, he run around in circles chasing his own tail. Anyways, me and the apprentice went there for half a day and sorted it out. The root of the problem was a ballast, but while we were there we found problems on six out of nine lamppost lights and two out of five wall lights. Ranging from lamps, starters and photocells. What got me was how he never noticed the water in the bottom of three lampposts. It gave me great joy to take these pictures and stick them to his forehead…lol

 

[Dave OCD]

Great idea with 'drain holes' but now the columns will rot away from the holes 10 x quicker.

 

[benji]

That L-N reading might not be the lamps ,remove fuses from cut-outs switch on if it does,nt trip mcb refit fuse one at a time till it trips . Is 50 megs on an old micc a bad reading?

 

[Dave OCD]

50 Megohms is a very good insulation resistance on any cable.

 

[davesparks]

Is 50 megs on an old micc a bad reading?

Depends what you're testing really?
A 50megOhm IR is a good value wheras a 50megOhm CR is a bad reading

 

[Kamikaze]

That L-N reading might not be the lamps ,remove fuses from cut-outs switch on if it does,nt trip mcb refit fuse one at a time till it trips . Is 50 megs on an old micc a bad reading?

I admire your benevolence but I’m pretty sure the BANG test is not a recognised or ever recommended method of fault finding in the regs book. Current and all previous editions included. Although, if you can find it in there, let us know because that information will be like gold dust.