All fuses blowing!

[Proper Job]

Hello
i have seen a first today.
i am working for a highway unit and have had to go and solve a fault .
I he I can explain this correctly as the wiring is in a bad state with a few under ground t joints to make it a bit more difficult for testing. So. The cable supplying the line of lights is a three phase cable with the yellow cable possibly being jointed under ground and going off to some lights that are not causing a problem. The cable continues on for about 150 meters. There are 8 lights within that 150 metres on the red and blue phase.the cables appear to loop in and out of the columns but may be jointed under ground, any thing is possible.and in the column is a mini is a double pole isolator and fuse carrier.
the problem is that at the last six columns each of the fuses have blown and I have yet to find out why.the main fuses feeding these circuits are ok. I have insulation resistance tested one of the cables from the column fuse to the light which turned out to be clear. I will test more tomorrow ,am sure it is something to do with the neutral being loose but would appreciate any advice.
thanks

 

[electric avenue]

Sounds like fun, not up on high way stuff but if it were me i'd look at the neutral too , any 3 phase line conductor normal 230 v between it and neutral shoots up to 400v if the neutral is not connected. Had rarther alot of super bright smoking lights when a neutral had been pulled out on a 3 phase comando plug one time.

Worst case just break it down ad check each bit of cable at a time.... ahhh the joys of fault finding.

Good luck

 

[Farmelectrics]

Split the circuit ant test till you locate problem takes as long as it takes and some more

 

[Des 56]

Seems like a lost neutral to that set of lights,2 different phases and a common lost neutral

 

[i=p/u]

R1 and r2

 

[Farmelectrics]

Simple continuity test will solve this

 

[Proper Job]

As well as r1 and r2 readings ,am I right in thinking that I should get the Rn readings also as they will tell me where the bad neutral is .
thanks again everyone for the help

 

[ackbarthestar]

the problem is that at the last six columns each of the fuses have blown

As others have stated. You have what appears to be a loose neutral and the voltage is swinging around from 0 - 500v

Try continuity testing from the 7th column from last .

 

[Proper Job]

On Friday I tested the three phase of that cable and every thing came back clear. The lowest ir reading of one of the phases was 8 meg ohms. I did r1 r. 2 readings and also r2 Rn and all readings were very healthy. As it was towards the end of the I just linked from one working light column to the drop feed of the first of the lights that is out and once I turned it on it blew the fuse.. Which is making me think that the last 6 lights have been damaged due to a surge or something, will test further when we I get back to that job, thanks for the advise about the loose neutral.

 

[Guest55]

what sort of lanterns are they ?
the latest street lights have radio signal control for switching and also electronic dimming capability.
in other words there could be a group of lights , fitted from the same batch , that are duff.

you've proved that the underground swa is sound and the section fuses dont blow so i'd be looking closely at the lights.
do a IR test between LN&E to the t&e cable going up to each lantern , should show up any weak electronics

 

[Proper Job]

I have done an ir test on each of the drops between l and earth then neutral earth. All came back clear.
how would test between live and neutral connected toghether tested to earth?
Cheers

 

[Guest55]

doesnt matter as you've tested L & N to E earth separately so its done already.
seems a very strange fault i must say.
how long before the isolator fuses blow after changing them ?
a day ? a week ?

 

[Proper Job]

As soon as I put a new fuse in at the base of the pillar it would blow immediately.

 

[Guest55]

are these street lights new or existing ones that have worked ok previously ?

 

[rattlehead85]

I would advise you check the control gear and photocell switch on each lantern. If these are old fittings that have not received correct maintenance there is a possibility that they may have moisture ingress or corrosion of the lamp internals. When you replace the fuses the photocells will kick in for a couple of mins which may be what induces the fault to the base of the columns hence blowing the fuses. An IR test may not highlight this as the fault is induced once the photocell operates.