Residual voltage on pannel

Re: Residual voltage on pannel

A safer way to test for ground faults is to take a 7-40 watt 120 VAC light bulb and clip it to between ground the a terminal lead. If it glows (or pops), you have a ground fault. If it does not light (and you measure close to zero volts--both AC and DC), you are OK.

One warning... I do not know how a, for example, Xantrex GT 3.x Grid Tied Inverter detects ground faults in an array... I would suspect that if you do the spark test (or even a large light bulb), you may cause it to ground fault detect and pop its internal ground fuse.

http://www.xantrex.com/web/id/172/p/docs/pt/25/product.asp

From reading the Xantrex manual (above), it appears that they place a 1 amp fuse between ground and the Negative Lead (or Positive Lead in the POS Ground option model). I guess, if the opposite lead is grounded and more than 1 amp of current flows, the fuse is popped, and the unit detects the fuse terminal voltage is not zero any more, the unit shuts down.

So, in this case, it is very possible that you will have up to 600 VDC (at 5-10 amps) between the Positive DC Solar terminal and ground if connected to a Xantrex GT 3.x inverter (for the "standard" negative grounded inverter).

If you have disconnected the inverter from the solar array (or possibly have another brand of inverter that does not ground the solar array), then using a DVM to measure the DC (or possibly AC) offset voltage is not accurate because the DVM has Megohms (or more) of resistance. Placing a small 120 volt light bulb will safely and properly ground the floating circuit and show you if you have a ground short somewhere (this test is commonly done on -48 VDC telecom/switch power systems to check for improper grounding--use a meter to see if the voltage goes to zero, or stays non-zero). By the way, to be totally safe, make sure that, if you are using a DVM, to measure both AC and DC voltages--most DVMs will show zero AC volts when connected to a DC power source, and zero DC volts when connected to AC power (you don't know if your short is a DC to ground or an AC to ground fault in some cases).

The Big SPARK method is not safe, and can also be misleading... For example, many supplies have some pretty hefty filter caps and you can draw a quick arc as you short out the charge on a filter capacitor. The light bulb, on the other hand, provides a constant (and limited) load and will quickly discharge the cap, or simply glow (or pass current) if there is a source of power present.

Lastly, if you have a ground faulted battery bus and try to draw a spark test (short), you will have a very nasty surprise if the wire welds itself to ground and catches your home on fire.

Sorry for the long post, but there are so many possibilities out there (grounding, equipment, possible failures, wide variety of voltages and currents, etc.), I would suggest that you get professional help (as in electrician :-D ) if you don't know exactly what is going on in the system you are testing. It can be very dangerous.

-Bill