Residual voltage on pannel
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I just wired up two pannels - a 120 W Kyrocia and an 80 W Shell - in parallel for a 12V system. While checking my connections and polarity etc. before the final connection, I measured 0.25 V between the +ve side of the pannels and the ground on the pannels, which is connected to the aluminum frames. Is this normal?I went ahead and hooked up the array to the charge controller and hope nothing bad happens.
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I have come across that quite a few times measuring 100's of volts from either polarity to ground. I take the wires and touch them to the ground, if you get a big plasma arc you have a ground fault, if nothing noticable happens, don't worry about it.
sometimes dmms will give erroneous readings without being connected or 100% connected. nothing to worry about. if it were 100% connected and you got that reading then you'd have a problem.
I keep running into high voltage from either polarity to ground, but there are no ground faults. The rack has full voltage, but there are no significant amps. Short either side, preferably both one at a time, to ground. If you have a big plasma display, you have a ground fault. That is the only way I have found that works. Big spark = Big Amps
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.
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.
Thanks for the comments. The 0.25 V is measured between +ve and gnd with the pannel disconnected. Just the sun shining on the face and no wires connected to the terminals. It seems to me that this is not an indication of a problem.
Serious Arcing! If it does this, STAND BACK! http://www.citlink.net/~pata/Solar/500kV_Switch_Opening_Under_Load.mpg