3 Phase System

[peter_k800]

Hi all, Got a call today to a warehouse where there is a 3phase system. Its a drycleaners and they have 2big drycleaning machines and 4large ironing table things all on 3phase. Then some sowing machines and standard single phase washing machines and tumble dryer and standard things like 2 computers and stuff in a canteen.

Anyway the prob is that every now and again one of the main fuses blows and the only thing on that phase is the 3phase machines and a couple of lights.

Any ideas on why this might be happening or how i could find the problem, only have a standard multi-meter.

Thanks in advance.

 

[Eggster71]

Beg borrow or steal an insulation tester mate.
It sounds like one of the phases is up the swanny. If it's the supplier's fuse going you'll probably be looking at either damaged tails or a fault within a DB or isolator. If you know what size the main fuses are then make sure you fuse down any fused isolators. Had a damaged underground submain a few years ago which took out the board's fuse and it turned out the switchfuse was rated higher.
Also check the tightness of ALL the terminals as you work through, look for blackened insulation as signs of overheating.
Watch yourself if you're new to this game(just a guess based on you only having a multimeter), you don't wanna be messing with that kind of gear unless you're SURE it's safe. 400V hurts.....a lot....

 

[peter_k800]

Its the main 63amp switch in the fuse boad that is blowing not the supplyers fuses, Would that mean its a prob within the system?

Insulation resistance tester, how do i test with that again head is fryed these days? Could damp-ness be a prob in the machines, as they only moved all the machines into this ware house about a month ago and the day they moved it was a bit wet could this cause probs?


Any other info would be greatfully helpfull. Thanks again.

 

[Eggster71]

Damp may be an issue but to be honest that should knock out individual circuits. IR test is 500V DC between whatever you're testing. Phase to phase, phase to earth , phase to neutral, neutral to earth. Test is done dead!!!
Make sure there is nothing connected either or you'll get false readings and also risk damaging circuitry/ equipment. My money is still on the tails or db being at fault though.
It's also not beyond the realms of possibility that is a pure and simple overload. If it was designed as a warehouse there was probably only allowance for lighting and smaller loads.
You'll need to test this with a clamp meter.

 

[peter_k800]

db ??
What kind of result would i be looking for again doing this test? In doing this test i can just test as far ar the isolator to machine is this right? Is it ok to test through MCBs?

The owner said to me the main fuse in there old building was 63amp too so dont think it would be an overload as its all the same equipment.

Thanks

 

[Eggster71]

DB is distribution board. You're after a reading in the millions of ohms range. Isolators off. All current using equipment isolated or removed. You can test through MCBs but if you suspect a fault I'd go a circuit at a time.
No offence here but if you're not completely confident you should really get into a mess!
You'll be much better getting someone in, they'll have the equipment needed, should be used to the procedures and most of all should have insurance in case something smelly hits the fan!
Test meters aren't cheap and aren't widely available for hire, it'll probably be cheaper to pay a sparky in the long run!

Have a quick look here: http://www.electriciansforums.net/e...lectricians-your-area-local-electricians.html

Best of luck!

 

[peter_k800]

Ya might get someone else to do testing as i dont have the equipment and havent done it since i qualified.

Each machine is on its own circuit, New 6mm and 10mm swa cable used for the circuits 10mm just for the two big machines.

Thing just has me baffled!!!

 

[Eggster71]

Have a look at the tails and terminals initially, you never know it might just be staring you in the face!

 

[peter_k800]

The supply tails feeding the board or the tails coming out of the main switch? Looking for blacking or melting is it?? Thanks again.

 

[Eggster71]

The supply tails feeding the board or the tails coming out of the main switch? Looking for blacking or melting is it?? Thanks again.

Anything before circuit breakers. Check the connections, carefully with a decent driver, what you're looking for is a possible fault between the main supply fuses and the outgoing circuits, check for "nicks" in cables as cables enter and leave trunking/ boards etc also within boards, it's been known for folks to pry cables into place with all sorts of sharpish utencils. Blackened and brittle insulation is a good indicator of a fault, as is a smell of burning and brownish deposits around terminals etc. If you have trunking etc check around entrys and exits for burn marks, damage to insulation.

 

[Sintra]

This also could simply be overload.

Check current with clamp meter on each phase at device that is tripping with appliances turned on.

 

[promaxia]

i would consider the design of the system.it sounds like the system is overloading not short circuiting.if its a particula fuse blowing all the time the check the load balancing across all the phases.on a normal working shift get your hands on a tong testers and measure the amps across the brown,black and grey phases and youll find that its the single phase supplies is causing that phase fuse to blow.

 

[boatnik1]

I know this might be a bit basic but worth looking at, and one I've come across on occasions. Since there was recent changes to the circuits, is there a possibility that the fuse that keeps blowing is being replaced with a standard fuse and not a motor rated one.