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Topic Title: Varistors giving problems on voltage test
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Created On: 04 July 2011 10:32 AM
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 04 July 2011 10:32 AM
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Avatar for dougflorence                                      .
dougflorence

Posts: 74
Joined: 25 July 2008

One of my clients has introduced some varistors into his equipment line to earth to improve immunity to voltage spikes to meet EMC requirements.

The varistors conduct at 370VDC for a 230V nominal AC supply, so we now get a low resistance to earth when we do a 500V test for compliance with EN60204.

We don't particularly want to specify that the varistors are disconnected when the equipment is tested when it is installed and on subsequent routine inspections.

Has anyone else come across this problem and can they suggest a solution?
 01 August 2011 01:16 PM
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philt74

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Joined: 25 July 2008

No expert in the testing standard you mentioned, but ...

You could the connect the varistor across the contactor coil, A1 and A2. Not between A1 and PE. If the control voltage neutral line, (hopefully connected to A2 of the coil), is linked to ground at the transformer end then you can break the nuetral/earth link before testing. If there is no link to disconnect, you could just disconnect the control voltage L & N supply wires from the source terminals.

Hope that helps/makes sense.
 01 August 2011 01:28 PM
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seeker

Posts: 319
Joined: 10 March 2007

Originally posted by: dougflorence

One of my clients has introduced some varistors into his equipment line to earth to improve immunity to voltage spikes to meet EMC requirements.



The varistors conduct at 370VDC for a 230V nominal AC supply, so we now get a low resistance to earth when we do a 500V test for compliance with EN60204.



We don't particularly want to specify that the varistors are disconnected when the equipment is tested when it is installed and on subsequent routine inspections.



Has anyone else come across this problem and can they suggest a solution?



Test at 250 volts to earth. Read the note below table 61 in BS7671
 01 August 2011 07:03 PM
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alancapon

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Joined: 27 December 2005

Originally posted by: seeker
Test at 250 volts to earth. Read the note below table 61 in BS7671

I agree. This note is still below the table in amendment 1 of BS7671:2008, so is still a valid answer.

Regards,

Alan.
 09 August 2011 11:30 AM
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eclectica

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We have this problem all the time with autoclaves and other lab equipment. The PSUs are highly filtered (common and diff) and have MOVs everywhere and capacitors to earth so that the control side does not lock up with an electrical spike comes along.

As Alan suggests, PAT test at 250V is the only way we've found around it - and label it clearly (sensitive equipment - I think that's in BS7671) to avoid problems for another PAT tester in the future.

Regards
E.

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 23 June 2013 05:10 PM
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Jrzluna

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I have the same problem with a Varistor AVX VCAS040218X400 it is a SMD component, it handles a voltage around 2.8V but after a time of use it reduce the voltage to 2.4V seem to be leaking current, we though the problem is related to flux contamination as far as after resolder the component it works perfectly. the overheating is activating again the component, the missing here is what is affected by the heat, component, flux, or PCB. So leaking seems to be a normal varistor behavior.
 23 June 2013 05:10 PM
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Jrzluna

Posts: 2
Joined: 23 June 2013

I have the same problem with a Varistor AVX VCAS040218X400 it is a SMD component, it handles a voltage around 2.8V but after a time of use it reduce the voltage to 2.4V seem to be leaking current, we though the problem is related to flux contamination as far as after resolder the component it works perfectly. the overheating is activating again the component, the missing here is what is affected by the heat, component, flux, or PCB. So leaking seems to be a normal varistor behavior.
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