The problem I always had with safety gaps at pole pig power levels was that once they did fire, they often didn’t quench, hence my opting for an arrester. Arresters won’t burn out if they are properly rated with a generous ceiling voltage. But otoh, following Jim’s logic, I’ve never had a pole pig to die in Tesla coil service, whether or not it was ‘protected’.
David
Sent from my iPhone
> On Nov 27, 2019, at 2:22 PM, Steve White <steve.white1@xxxxxxxxx> wrote:
>
> My transformer doesn't have a BIL rating on its nameplate but I assume that it is similar to other 10 KVA pole transformers. I am currently using a horn gap. It has rarely fired. I've been operating this coil for 3 years now with no maintenance required. I've had these MOVs sitting around so I thought that I would put them to use. It sounds like I should just stick with my safety gap. MOVs have always bothered me somewhat anyway because they are sacrificial in nature. "If it ain't broke, don't fix it".
>
> Steve White
> Cedar Rapids, Iowa
>
> ----- Original Message -----
> From: "jimlux" <jimlux@xxxxxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Sent: Wednesday, November 27, 2019 10:58:09 AM
> Subject: Re: [TCML] MOVs for pole transformer
>
>> On 11/27/19 8:46 AM, Steve White wrote:
>> I have about twenty 1800 volt MOVs that I bought several years ago. I am considering replacing the safety gap of my pole transformer powered RSGTC with an arrester made from these MOVs. The RMS output of my 10 KVA pole transformer is 14.4 KV. What peak voltage value should I use for the MOV string?
>>
>
> I'd stick with the safety gap.
> MOVs die a bit with each overvoltage. They start leaking more and more
> current, until they melt or catch fire from the dissipation.
>
> A classic design error is to put the voltage on something like a 120V
> power "surge protector" a bit too low (like 180V or 200V), so they keep
> taking hits. Then they get hot and burn up.
>
> Your RMS voltage is 14.4, so the peak voltage is 20.4kV. A 30kV string
> might be ok. But, consider that a pole transformer is designed for
> transients (the BIL might be 90kV or higher) - so the question would be
> "what is the MOV protecting?"
>
> https://www.powerpartners-usa.com/wp-content/uploads/2016/01/single-phase_product_spec_sheet.pdf?x57229
>
> Safety gaps are essential on NSTs - they are designed to be low cost,
> and at the ragged edge of failure, insulation wise.
>
> On a pole transformer, I think a safety gap is more of a diagnostic tool
> - if your safety gap is firing, something in the system is out of tune.
>
>
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