UPS shock protection

[lyledunn]

On UPS systems operating at mains voltage, what shock protection measures are included to provide protection against indirect contact? Regards,

Lyledunn

 

[dpc]

I'm not clear on your concern. Contact with what? Wiring from UPS to equipment should be no different from any other wiring.

 

[peebee]

Are you talking about the batteries? It's always kind of bugged me that wet cell batteries are often installed with no covers over the buswork. I'd strongy recommend plastic covers.

Otherwise I agree with dpc, the wiring should be installed about the same as the AC system.

Of course there's always the old rule of thumb: "one hand in pockey, no get shockey."

 

[dydt]

'mind if you elaborate your question..it is not too clear..shock from what?

dydt

 

[lyledunn]

To be succinct, what I really mean is can one receive an electric shock from a UPS other than by touching the poles directly.Consider the output of a large single phase UPS operating at 220v where the main supply has failed. Thus we are now on battery power. What protection is afforded to prevent say, a double fault occurring where one pole is exposed and a person touches it, the second fault is on the other pole to earth thus creating a shock hazzard? Regards,

Lyledunn

 

[peebee]

It's still not quite clear what you are talking about, and why it would differ from any other installation. Are you concerned about contact with the batteries? Yes, that is a valid concern, and for that reason I strongly recommend insulating covers for batteries installed on open racks.

 

[IRstuff]

You should have the same protection that the mains have, up to including a AGFI circuit.

TTFN

 

[Graeme38]

lyledunn,

The Electricity at Work Regulations should prevent someone from accessing live busbars. In the MoD we used to operate a Permit to Work System where any person working downstraem of a UPS had to be issued with a Permit to Work. The Authorised Person is responsible for writing a safety program showing points of isolation etc ensuring that the system was properly isolated and safety lock fitted. Thus if you are to comply with the Electricity at Work Regulations this situation should not arise, other than say when testing. We tended to treat LV UPS systems the same as fully automatic standby generators and HV (1000V +) in the same manner.

 

[kitha]

You better read articles on Isolated Grounding as commonly called in North American region. Most of the UPS have an isolation transformer. As per most of the electrical codes in the world a GF(ground fault detection) breaker protects the primary side of the UPS or in European standards a RCD/ELCB along with a breaker protects short circuit ground fault(shock protection). As the secondary side is isolated from the primary theres no need of worrying about shock protection. Therefore basically it is the particular installation standard that prevents electrical shocks.

 

[peebee]

Kitha, I think some of your statements are possibly misleading and dangerous:

"As per most of the electrical codes in the world a GF(ground fault detection) breaker protects the primary side of the UPS or in European standards a RCD/ELCB along with a breaker protects short circuit ground fault(shock protection)." -- that GFI protection is for equipment, not personnel. You can certainly get a healthy shock off the secondary of a UPS.

"As the secondary side is isolated from the primary theres no need of worrying about shock protection." -- Every two-winding transformer provides isolation between the primary and secondary. And many of them are quite capable of giving you a shock. Many UPS systems are also quite capable of giving you a shock.

 

[kitha]

First of all think this way. If the installtion of UPS poses a shock hazard then all the UPSs would have been rejected by all the electrical codes. That is not true there are numerous ways to protect shock hazards. Before the installation of UPS, must think of the earthing scheme of the system. It can be TT, TN-C/TN-S or IT as per IEC standards. This data should be given to the manufacturer for proper selection. If galvanic isolated UPS is installed the installer can decide the earthing systems of the primary and the secondary of the UPS. I will briefly explain what happens in each of other cases.

TT(Primary)/TT(seconday)- The RCDs/ELCBs installed at the primary(field work) gives the shock protection for primary insulation failures. When the invertor works, as the neutral of the invertor closes through a contactor(manufacturer provided) to the ground the shock protection is detected by RCD/ELCBs installed(field work) to the output circuits. Therefore RCDs at the primary and secondary circuits are necessary.

TN-C/TN-S - As commonly known the circuit breakers are properly selected to clear the earth faults. Therefore no RCDs/ELCBs are necessary at the primary. RCDs/ELCBs are necessary at the secondary and operate due to the same closing contactor earthing the neutral of the inverter to the primary transformer bonding conductor which is inherently earthed.

IT/IT - In all the isolated grounding systems earth leakage current detection systems are used and isolate the circuits upon exceeding the pre-determind values.

Therefore these protections are provided. But the user has to inform the manufacturer what type of earthing scheme is going to be used and should select the protective devices with proper coordination.

Why I talked about a GF breaker is to understand the people in north American region. They don't commonly use RCDs/ELCBs as per IEC. Instead they use GF breakers which give short circuit, overload and earth fault protection as well.

The book "Electrical installation guide according to IEC standards" published by Group Schneider explains the above phenomina very clearly. If somebody is interested I can send by mail the relavent pages.

 

[kitha]

I would like if somebody comments on my explanation. or add something new to the shock protection schemes of UPS.