This stray capacitance previously mentioned is referenced to GROUND (L-G,
N-G).
L-N loads should not present an issue to a GFI, so be careful with your RF
bypassing
capacitors, they could cause you grief. Just sayin',,,
On Wed, Mar 12, 2014 at 9:19 AM, David Sharpe <sparktron01@xxxxxxxxx> wrote:
> Yep, any stray capacitance to ground will cause nuisance tripping of a
> GFI. If you assume
> 0.005A (5mA / UL limits) for GFI, at 120VAC is 24K impedance, which
> calculates out as
> ~ 0.1 uF. I tested with a MMC tappable capacitor bank, and GFI in my
> house tripped at
> 0.09 uF. This is also reason why RV's can not use a GFI protected shore
> tie, any UL listed
> caps between H/N and N/G will cause nuisance tripping (microwave, AV
> equipment,
> HVAC). Been there, done that, and got spanked by it...
>
>
> On Fri, Mar 7, 2014 at 7:17 PM, David Speck <Dave@xxxxxxxxxxxxxxxx> wrote:
>
>> Jim,
>>
>> I'll concede your points.
>>
>> However, if you are having spikes big enough to deplete MOVs in a short
>> time, the rectifier diodes won't stand a chance.
>>
>> It can be prudent to put a suitable fuse in series with each MOV to guard
>> against short circuit failures. Of course, you have to watch the fuses,
>> 'cause if they blow, then you lose the protection of the MOVs.
>>
>> Do any TC builders run their coils through GFI circuits? To my
>> understanding, a TC is a granddaddy of a ground fault when in normal
>> operation.
>>
>> Dave
>>
>>
>> On 3/7/2014 6:25 PM, Jim Lux wrote:
>>
>>> On 3/7/14 2:02 PM, David Speck wrote:
>>>
>>>> Dave,
>>>>
>>>> I would also place suitably rated, beefy MOVs across the input and
>>>> output leads of the bridge, and possibly across each individual leg of
>>>> the bridge. Rearranging the wiring have caused the system to pick up RF
>>>> in a new way that is blowing the bridge.
>>>>
>>>
>>> MOVs in a system where there are spikes all the time is a recipe for
>>> disaster. Each time they clamp, they die a little, and conduct a bit more
>>> leakage current. I learned that on the 3 phase motor drive scenario.. Kill
>>> a big fat MOV in hours.
>>>
>>> If you want a clamp, use a gas tube type or a semiconductor device. They
>>> can clamp millions of impulses.
>>>
>>>
>>>> I'd also place suitably rated 0.1 uF ceramic disc caps across each leg
>>>> of the bridge to bypass stray RF. Electrolytic caps are too slow to
>>>> protect against RF at TC frequencies.
>>>>
>>>
>>>
>>> 0.1 uF will result in some noticeable leakage at 60 Hz. It's about 27k
>>> impedance, so you get about 10 mA. Probably not a big deal in this
>>> application, but it's more than enough to trip a GFCI.
>>>
>>> Series L might be a better approach to filtering.
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>>>
>>>
>>>
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>>
>
>
>
> --
> Dave Sharpe, TCBOR/HEAS
> Chesterfield, VA USA
>
> Sharpe's Axiom of Murphy's Law
> "Physics trumps opinion!"
>
--
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA USA
Sharpe's Axiom of Murphy's Law
"Physics trumps opinion!"
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