What applications are 10ma RCD's required in?

[eveares]

Where's your isolating transformer?

as a extra precaution along other things like isolation transformers...

 

[stillp]

Where's your isolating transformer?

as a extra precaution along other things like isolation transformers...

So, if the supply is isolated, what extra safety will result from using the 10mA RCD?

 

[eveares]

But what if one decides not to use the isolation transformer for what ever reason or if the isolation transformer develops a fault.

 

[PBC_1966]

IIRC, in the US, their general use RCDs (GFCI) are rated at only 5mA with an allowable trip time of 25mSec.

That's right. And the nominal 5mA specification is really 4 - 6mA, i.e. the unit must not trip at a sustained 4mA but must trip at 6mA max.

 

[Risteard]

30mA RCDs which take anything like 40 ms to operate at 1 x IΔn

They have 300mS to operate at I delta n if a BS EN type, or 200mS for older BS types. 40mS is only for 5I delta n where the RCD is being used to provide additional protection.

 

[PBC_1966]

But what if one decides not to use the isolation transformer for what ever reason or if the isolation transformer develops a fault.

There are so many "what if" scenarios for something like this. I've used an isolation transformer for years for working on older live-chassis equipment, but you still have other hazards present which neither an isolation transformer on the supply nor an RCD/GFCI on the supply will reduce even if it's not live-chassis apparatus: HT supplies of hundreds of volts or several kilovolts in valve TV sets, transmitters, etc.

 

[stillp]

But what if one decides not to use the isolation transformer for what ever reason or if the isolation transformer develops a fault.

What if the RCD develops a fault? Why would the isolated supply not be used?

 

[SimonH2]

Meanwhile, away from the willy waving ...
I have a 110V wall mounted transformer sat in the corner of the garage that came out of a school when it closed down. Seemed like a good idea at the time. It's both centre tapped earth and has a current trip with several turns of the centre tap earth through it's sensing hole which IIRC takes the nominal trip current down to 10mA or could be something less.
At 6th form, IIRC they reckoned they had one or two items of equipment that would frequently trip the supply in the physics lab due to the sensitive nature of the protection.

 

[PBC_1966]

That's a problem with the the 5mA GFCI devices here. They might be fine to protect an outlet in a bathroom but try feeding a dozen outlets on a test bench thtough one GFCI and you'll likely end up finding it impossible to run certain combinations of equipment simultaneously due to the ground current (by design, especially in some older pieces of equipment).