[Info-vax] VAXStation 3100

[Michael Moroney]

David Wade <g4ugm at dave.invalid> writes:

>On 16/01/2019 20:41, Bill Gunshannon wrote:
>> 
>> OK, before I go screwing with it any more than I need too....
>> 
>> I fired  up the first 3100.  It has 7.1 (but I don't remember
>> any of the passwords.  Not a problem, I know how to break into
>> the box and reset the system password.) and 5.5 on it.  That
>> must go back quite a ways.
>> 
>> But here's the new problem.  I plugged an Expansion box in
>> the same outlet strip and connected them together. Kicks
>> the GFCI breaker!!  Disconnect the SCSI between the two
>> boxes.  As soon as I turn the second one on it kicks the
>> GFCI.  Either one is OK by itself, but the two together
>> kicks the GFCI.  Other than a wiring problem in the power
>> supply (I assume it would take one of them having the white
>> and black wires reversed in side the box) what else could
>> cause them to kick a GFCI as opposed to a circuit breaker?
>> 
>> bill
>> 

>Its almost certainly not a faulty GFCI. Thats expected behaviour with 
>older equipment that has a slight leakage to ground. GFCI's are set to 
>trip at certain leakage current, in the UK its 30Ma but may be different 
>in USA.That is if the current on the live is more than 30ma different to 
>the current on the neutral it will trip.

>So if each box leaks 20Ma to ground they will work fine when connected 
>separately. Connect them both to the same strip. Leakage goes above 30Ma 
>the GFCI will trip.

>You always get Earth Leakage in computer equipment, especially in older 
>equipment because they often have filter capacitors that connect the 
>live and neutral to ground, to filter out spikes.

>If these capacitors become leaky then they conduct more than the design 
>spec.


Not simply capacitors, equipment, esp. things like UPSs or any "surge
protectors" will have MOVs or other surge supressors between both lines
and ground. This is to protect against "common mode" surges, such as when
a nearby lightning strike induces a high (and nearly equal) voltage in
both of the power conductors. This can fry things between the equipment and
something with an actual ground.

If the MOVs are leaky and conduct a little, a GFCI may trip because of
it.

What you wrote is correct other than the fact that the things you call
capacitors may not be capacitors, they are surge supressors of some
sort. Especially since many MOVs look like disk capacitors.

https://en.wikipedia.org/wiki/Varistor