Contactor problem - possibly

[pjcomp]

Here's the scenario:
Big house with detached granny annexe. Three-phase supply. Single phase supply by buried SWA to annexe for power and light. Separate three-phase supply by buried SWA for the annexe night storage.

The clock for the NSH is in the main house. The 3ph goes to a fused switch (three 32A fuses, big chrome handle on the outside of the box) then to a 3ph contactor, then via SWA to a 3ph CU in the annexe. Each phase goes to two storage heaters, each heater on its own circuit and MCB.

Now the problem:
The night storage in the annexe worked fine at Christmas. It was switched off after granny went home, then switched on again at Easter for the next visit. Didn't work.

The three 32A fuses had all blown.

Replaced the fuses, the night storage was switched on again. And the fuses blew again.

Nothing tripped in the annexe consumer unit. The contactor appears OK, unjammed, slides freely.

All circuits checked, no shorts, no problems found. 5a fuse for the contactor is intact. No problems on any other circuits in the house or annexe.

Any suggestions what might be knocking out the fuses? The presence of the contactor in the equation is throwing me a bit - I've not had much contact with them and not too sure why it's been used in this case as I wouldn't have thought the loading required it. Someone with more experience of them might put me right ....

PJ

 

[telectrix]

looks to me like some IR testing is required . from 32A fuses to contactor to CU. ifall 3 fuses are blowing, i would think the fault is before it splits at the CU to each heater.

 

[chopper76]

Sounds like its after the contactor but before the sub main cu, guessing damaged swa submain to take out all three phases

 

[Gage]

Only thing the contactor could do for this to happen is not energise properly for whatever reason, and essentially form a loose connection. But as said above it doesn't sound like its this.

 

[SteveEmartin]

Try testing the 3 poles of the contactor to make sure they have not shorted between phases internally. I have experienced this before..

 

[Hawk81]

An insulation resistance test would be the next step.

 

[itselectric]

As you said they are night storage heaters so contactor is controlled by time switch to switch heating on and off. As for fuses blowing I agree with prevoius posts.
The only other thing I can think of is dampness if annexe has been left without heat for a while. the older nightstore bricks were like blotting paper, they absorbed water. I've not installed these heaters for a long while so dampness may not come into it with new ones.

 

[chopper76]

If that we're the case I would expect the individual mcbs for the night stores to trip before the 32a fuses on the submain to blow

 

[Knobhead]

Given the OP, a qualified electrician is required.

Never come across a switch fuse made by "big chrome handle" before.
If you can’t give any useful information why ask the question?

 

[onions1066]

Given the OP, a qualified electrician is required.

Never come across a switch fuse made by "big chrome handle" before.
If you can’t give any useful information why ask the question?

no real need for that tony

 

[vaughant]

Is the contactor kicking in the moment you throw the isolator?

That will quite easily blow fuses as I had a very similar thing (albiet with a heavier load) when the lad had wired up the stop start with two n/c contacts hence the second you threw the isolator the motor tried to run.

remove the 5a fuse to the contactor or ensure that the heaters are not being asked for i.e contactor not energised and see what happens again with the fuses when you throw the isolator.

Then, making sure you can control the contactor on and off remotely NOT solely via the isolator , energise the contactor with the fuses in to see what happens again.

Could IR test first of course but a simple ohmic test will show up anything obvious basic checks to earth etc and with the power off test the contactor for resistance across the contacts, although this isn't conclusive of course, it does let you know if theres a major issue.

 

[Outspoken]

Here's the scenario:
Big house with detached granny annexe. Three-phase supply. Single phase supply by buried SWA to annexe for power and light. Separate three-phase supply by buried SWA for the annexe night storage.

The clock for the NSH is in the main house. The 3ph goes to a fused switch (three 32A fuses, big chrome handle on the outside of the box) then to a 3ph contactor, then via SWA to a 3ph CU in the annexe. Each phase goes to two storage heaters, each heater on its own circuit and MCB.

Now the problem:
The night storage in the annexe worked fine at Christmas. It was switched off after granny went home, then switched on again at Easter for the next visit. Didn't work.

The three 32A fuses had all blown.

Replaced the fuses, the night storage was switched on again. And the fuses blew again.

Nothing tripped in the annexe consumer unit. The contactor appears OK, unjammed, slides freely.

All circuits checked, no shorts, no problems found. 5a fuse for the contactor is intact. No problems on any other circuits in the house or annexe.

Any suggestions what might be knocking out the fuses? The presence of the contactor in the equation is throwing me a bit - I've not had much contact with them and not too sure why it's been used in this case as I wouldn't have thought the loading required it. Someone with more experience of them might put me right ....

PJ

PJ,

• Did you actually witness the storage heaters working at Xmas?
• What works internally or externally have taken place since Xmas?
• Have you checked the contactor to ensure L1 - L1 / L2 - L2 / L3-L3??
• Have you tested the SWA (3 Phase) to ensure it has not been damaged?
• Are you a qualified spark?
• What experience do you have with 3 phase systems?

Now I am sorry if these seem like questions insulting your professionalism or intelligence, but with all due respect, if you come on an Electricians forum and ask the question you did without even doing the basic checks then one must question whether you are actually competent to be working on any electrical circuit let alone a 3 phase one.

If you need to ask this question, then your not competent or experienced enough in my eyes and perhaps you should simply contact someone who can do the job.

 

[pjcomp]

Outspoken, thank you for your comments - now please read the original post: All circuits checked, no short, no problems found. So yes, I have come on here after doing the basic checks. Obviously I'm missing something or I wouldn't still have the problem, but then that's why people ask questions, to find answers.

Have I witnessed the heaters working? No, but I've no reason to think the customer would have let her granny freeze over Christmas.
No work has happened internally or externally since Christmas
Checks on the contactor - no. Thanks SteveMartin for pointing out the internal problems that can occur, which is the sort of help I was looking for.
SWA tested and checked for damage (see basic checks, above)
Yes
I have limited experience with 3ph systems. I stick to domestic and small commercial where 3ph is rare and this sort of problem is new to me, even after all these years. Ask questions, learn from the answers -it's called education.

Thanks to everyone else who posted useful and constructive ideas. I'll let you know when I find the answer

PJ

 

[Outspoken]

PJ, look I apologise if my stern tone caused you some offence, that was not the intention, but all too often I have encountered situations like this and upon investigation found that all required tests had not been done or done properly, that people gave inaccurate/misleading information or downright lied, in the case of customers that last one.

Clearly for all three fuses to blow simultaneously there has to be fault that is common to all three phases as you do not have three phase loads attached. This leaves four possible causes that need to ruled out before looking at other routes.

1. The cable acting as the sub-main ( You say you tested this, but did you fully ir test the cable?)
2. The contactor connections and body.
3. The link from the contactor to the db.
4. The db where the 3P is split to supply the single phase outlets.

If you check all the above properly and still have no fault you need to look further into the circuit to ensure that phases and neutral have not been crossed, I am assuming, as you did not mention it, that the 3 P supply has a neutral and they have not used two phases per heater? If your not sure I suggest che king as I have found this done several times over the years.

Jim

 

[vaughant]

Pj did you try my suggestion?

Helps eliminate it in a logical way,very rarely have I heard or even experienced a 3 phase main incoming swa "breaking down" it's almost always at the load end.

Let us know how you get on.