DC-only Generator: Shock Hazard Questions

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robertcoats

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Hello from Honda Generators:

I work in the Product Planning group, and we have a project involving a prototype DC-only generator. The unit produces 39.5 or 52.5 VDC @ 2800 watts and has a chassis ground lug, which is grounded properly when the generator is operated.

Due to a bit of language barrier between my group and our development team in Japan, I'm trying to independently determine what (if any) hazard there might be to an operator if they made contact with one or both of the output terminals.

So, here are the specific "what if" questions:

(1) What happens if the operator and generator are on wet ground, and the operator's wet, bare skin makes contact with the +52 VDC lead (only)? How about the -52.5 VDC lead (only)?

(2) What happens if the operator's wet fingers make direct contact across the +52 VDC and -52VDC output terminals at the same time?

(3) What real-world hazard(s) are/is there with human contact to DC voltages at these levels?

Thanks!

-Robert
 
Re: DC-only Generator: Shock Hazard Questions

I design -48 VDC plants, and may be able to help.

(1) What happens if the operator and generator are on wet ground, and the operator's wet, bare skin makes contact with the +52 VDC lead (only)? How about the -52.5 VDC lead (only)?

A. It depends on whether or not one of the polarities is bonded to the frame and if the frame is in contact with earth. If you have one polarity bonded to the frame and the frame is in contact with earth, and the?operator touches the ungrounded polarity, he/she might feel a sensation to moderate pain. On the other hand if neither polarity is bonded to earth, nothing happens.

(2) What happens if the operator's wet fingers make direct contact across the +52 VDC and -52VDC output terminals at the same time?

A. It really depends on the condition and health of the individual. The symptoms range from no sensation to moderate pain. My experience is no sensation unless I am sick a sweaty, then I fell a sensation or tingle. The real danger is from shorting the terminals out with something like a screw driver or wrench and the flash that can develop.

3. What real-world hazard(s) are/is there with human contact to DC voltages at these levels?

A. Mostly flash hazard.

I assume your generator is to be used to supply +24 and -48 VDC systems. If that is the case then the NEC does not require these system to be referenced to earth or grounded, but is permitted. Typically in the Telco sector we do ground these systems as it makes it much more ecconomical and easier to employ over-current protection methods such as fuses and circuit breakers.

Hope this helps.
 
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