UPS shock protection

[lyledunn]

What measures are employed to provide protection against ground faults on the load side of an UPS? I am in the UK and I have found that with mains healthy, earth loop impedance tests taken on the load side of the UPS often results in relatively high impedance values thus often rendering reliance for shock protection on mcbs alone less than adequate. However, I am also aware that there is ground fault protection built into the UPS which rapidly reduces voltage to the load in the event of a fault to earth. What I would like to know is just how this operates and can it be relied upon to provide appropriate shock protection or is it just designed to protect the UPS itself?

Regards,

Lyledunn

 

[rbulsara]

In the US, Code does not require to provide 'shock' protection (GFI or RCD) on UPS circuits.

In fact Code only requires GFI for cicuits where hand held (portable) devices are likely to be used in a damp/wet area. I on the other hand see providing RCD on electronic equipment counter productive or more prone to nuisance trips. And any RCD/GFI more than 30mA sensitivity is useless for 'shock' protection.

I am also not sure why people in UK keep saying high ground loop impedance? Isn't UPS is grounded and a seperate ground (earthing) wire is run with each circuit? Specailly for critical applications where a UPS is necessary.

 

[ScottyUK]

rbulsara,

Differences in terminology again!

The earth loop impedance (of a final circuit) that us Limeys are referring to is the loop formed by the phase conductor and the protective earth conductor. This is different to the loop impedance of the phase and neutral conductors because UK practice is (generally) to use a smaller protective earth conductor compared with the phase conductor.

Were you thinking of a loop formed by phase conductor and, literally, the impedance of the earth back to either the earthing rod or distribution transformer (or UPS).

Clearance time of a protective device obviously depends upon the fault current, and the impedance of the final circuit conductors forms a significant part of overall loop impedance when in a utility-supplied situation. When power is supplied from a UPS, the fault level is already pretty low and the effect on fault clearance time caused by a high earth loop impedance (i.e. a long circuit, or one with undersized conductors) is exacerbated. In some cases the fuse or breaker simply won't clear, unless the UPS has a bypass off the utility line which can deliver enough current to operate the protective device.

 

[rbulsara]

Scottyuk:

Thanks for clarifying.

Just to share information, in the US the common practice ( for applications in data processing centers etc.) is to keep UPS close to the loads. For larger facilties, UPS is generally 480V output and combination step down tranformer/panelboard units are installed close to the load, so that 120V circuits are not longer than 100-125 feet (35-40 meters). Also the ground conductor for 20amp circuits (more than 90% of the circuits are 120V,20A), is the same size as the phase condcutor (#12 AWG).

My comments regarding not requiring a GFI/RCD are still true.

 

[jbartos]

Suggestion: A sample of the converter output protection:
Output Circuit Protection: Current Limit, Short Circuit Protection, Thermal Protection, and Semi-Conductor Protection

TVSS may be added. Visit

http://www.novaelectric.com/nova_freqcon2.html

etc. for more info