earth fault on a chiller

[Davey1988]

hello guys,

im currently having a dispute at work and was wanting some advice. A work mate of mine said that a device (chiller) which a industrial cooling machine for a welder. is supposed to have continuity to earth from neutral after i diagnosed it as unsafe.

im saying it is not supposed to interlink. if that was the case they would use the cpc as the neutral and possibly make the outer metal casing live.

it has a motor on it but its only 1 phase. build in capacitor with the motor. and a few valves and plc buttons and stuff.

i assume yes in some circumstances (e.g distributor tn-c-s) system they center tap the cpc at their end but from ours they remain seprate?

your thoughts?

 

[Marvo]

Yes and no. Yes, the CPC must not be a part of the return path for the load current. No because the CPC is functional as well as protective so under certain circumstances there may be an earth current flowing. Think surge arrestors or earth referenced communications systems.

Mega test the compressor to start with and mega test the entire machine at a lower voltage (250v) Establish where the the low IR problem lies.

 

[Davey1988]

How often would you say that neutral to earth is common. As I'm sure this ain't supposed to. It basically gives me the reason when uli put my continuity tester on the plug and activate the contactor inside the machine.

 

[Davey1988]

Can I damage circuit boards with IR, as I'm assuming the contactor I switched was controlled by a 24v circuit. As I also got the reading through this circuit as shorting L and E Also.

What examples of systems may The earth and neutral be interlinked?

 

[Darkwood]

If you have a control board as in your example where you have 24v (any voltage really ac or dc) then the output of the transformer or DC supply unit may have one leg strapped down to earth, this varies depending on the control arrangement and isn't always done. The control side of a system in this case is classed as an independent circuit and power source from the incoming power supply to the building or chiller in your case.

The BS 7671 does not cover control circuits like in your case 'a chiller' or other machinery etc etc .. this will fall under the LV Directive and probably found within the BS60204.

Be careful you don't apply the BS7671 to circuits that are not covered by the regulations... the BS7671 more or less stops at the power supply terminals on the unit or the isolator/socket etc on the wall.

 

[ian.settle1]

I have know people to test equipment and have said there was an earth fault only for there not to be as they havve forgotten totally to remove the equipment from the supply including the neutral and earth.

 

[Darkwood]

We could do with a list of the tests and results plus exactly where these tests were an where about on the circuit they were done.

 

[Lucien Nunes]

And, specifically, how and where the plant was isolated during the test. Although the chiller seems to be single phase the supply to the welder might be 3-phase, so any associated isolator might not break the neutral.

OP you are correct, N & CPC should never normally interconnect within equipment or installations after the consumer's terminals.

 

[Davey1988]

this incorrect??? Should it be linked to the end neutral not earth as a neutral return.

 

[Darkwood]

That TX is connected fine 230v and 0v volt is your input L/N and the output is grounded on one side - this is a standard set-up .. don't forget the output is isolated from the input with only a common earth between by the looks of it so any testing in the input side will not see the output grounding... having side that you shouldn't really be using an IR test through any control circuit and only to check the installation side unless you are fully adverse with what you are testing and how it works.

 

[Davey1988]

I have linked the neutrals together and removes the link to earth as this in my eyes is a quick fix. and now it runs perfect with no rcd tripping.

Before hand if I pushed the contactor so they closed. I could get a continuity from the casing to live and neutral.
I know it's only low voltage but no voltage is better to earth?

 

[Deleted member 26818]

This is not covered by BS7671.
There are various reasons why the neutral and earth can be linked.
With welders it usually to reference the neutral to earth keeping the neutral at zero volts.
This helps with the quality of the weld, and prevents the voltage on the neutral from ever being greater than the voltage on the live.

 

[Darkwood]

I have linked the neutrals together and removes the link to earth as this in my eyes is a quick fix. and now it runs perfect with no rcd tripping.

Before hand if I pushed the contactor so they closed. I could get a continuity from the casing to live and neutral.
I know it's only low voltage but no voltage is better to earth?

Sorry am I understanding you correctly ??? have you linked the 0v input (N) to the 0v output???

This is dangerous to do if so - the transformer is a standard isolating transformer with one side of its output grounded - this is how it should stay if thats how you found it.

Can you draw a before and after diagram or post a before and after pics so we can clarify what you are doing???

 

[Deleted member 26818]

Sounds like you have a break down in the insulation separating the TX from the supply.

 

[Darkwood]

Agree with Spin here ... what is scaring me is what has been done to rectify the issue, yes the RCD has stopped tripping but I really don't think you realise the danger you have created here if you have indeed coupled part of the input to part of the output of the TX - I would connect back the way you found it and leave it alone - tell the customer the chiller is faulty and needs an Engineer in.

 

[Deleted member 26818]

By linking the supply and output neutrals, you have now made the installation neutral part of the return path for the welding current.

 

[Darkwood]

By linking the supply and output neutrals, you have now made the installation neutral part of the return path for the welding current.

Looking at the pic this is just the control TX for the chiller not for welding but still it hi-lights the risks of doing this.

 

[Lucien Nunes]

What he said ^^.

OP, to amplify the above, you must recognise that you have breached the isolation provided by the transformer and made the ELV 24V circuit part of the LV 230V supply circuit. In doing so you have 'disguised' one fault by creating another. In the event of a supply neutral loss, the 24V control circuit could become live with 230. Undo your mod, stop guessing, study the circuit and find the real fault...

 

[Davey1988]

Cheers for the advice guys. Put back the way it was. I'll read more about it but does seem that something may be wrong with the tx.

Just can't figure out why they use the earth as a neutral By linking it out.

Also I'm getting a live and earth reading once the contactor activates. Surely the tx should isolate this link and all lv circuit so it should not pick it up?

 

[davesparks]

Cheers for the advice guys. Put back the way it was. I'll read more about it but does seem that something may be wrong with the tx.

Just can't figure out why they use the earth as a neutral By linking it out.

Also I'm getting a live and earth reading once the contactor activates. Surely the tx should isolate this link and all lv circuit so it should not pick it up?

They are not using the earth as a neutral! They are connecting one leg of the outgoing side of the transformer to an earth reference. This is the same thing as the star point of the substation transformer being connected to earth