Trying to get to grips with my first solar installation. The panels are in strings of 3 with 3 strings into a combiner box (9 panels) then into a Midnite Classic 150. Can anyone shed some light on why i measure 11 amps in the positive cable but only 4 amps in the negative return on many of my 3 panel string sections?
B;
Well, it probably is a wiring or user error. There is simply no way you can have less in the negative cable than the positive. or the other way around. Current is equal in all parts of a series circuit. Period.
Part of the current may be flowing through another circuit component or possibly you are not reading it correctly or the test equipment is to blame?
We really will need a much more detailed description of the wiring and how you are doing things to do anything but guess.
Tom
Quote from: VicB on October 10, 2014, 05:34:15 PM
Can anyone shed some light on why i measure 11 amps in the positive cable but only 4 amps in the negative return on many of my 3 panel string sections?
Where, exactly, are you measuring these currents, and how are you measuring them? Are you using a DC clamp ammeter?
--vtMaps
One possibility is one specific wiring error, namely connecting the negative string lead to a ground wire in more than one place.
If you have a negative grounded panel and CC combination, then the bond between the panel - and your equipment/racking/safety ground should be in one place only. Most commonly right at the inverter.
You do not want to have normal circuit current flowing through the racking, wiring raceways, or grounding (green wire, EGC) conductor.
A connection between the string - at the panel frame or racking would allow a fraction of the current to bypass the - wire.
As VicB asked, where are you making this measurement? If you make it on the actual - lead coming out of the bottom panel in the string, most of the ways in which the wiring could be messed up will not affect that measurement.
If you are measuring it at the combiner input, then there are a lot more opportunities for misteaks.
If the string - wires are connected together at the panel end and at the combiner and you are using a shunt type meter instead of a clamp meter, just the act of putting the meter in series with one of the wires will reduce the current through that wire, because the overall resistance will be higher than that of the remaining wires in parallel.
I have observed similar issues on my system; particularly when measuring current between my epanel and the batts and between the inverter and epanel. I chalked it up to a bad clamp on meter (I use the Sears meter that BB recommends on NAWS.) btw I just can't seem to get the hang of using the DC Clamp feature. The zero really throws me off.
If the goes-inta is the same as the goes-outa, then I am willing to bet that you are using a
DC current clamp on probe and that it just needs to be "zero'd" in each direction before
you actually clamp it over the wire or turn on the juice.
Those probes tend to act different in one direction than the other.
To check this, measure one way on your positive wire and then turn the probe
around and measure the positive wire again and see if both measurements
agree or are off by around 2 amps.
These issues are why I don't generally like magnetic current measurements, especially
for DC currents.
Of course, maybe the problem is something other than the orientation of a DC current probe ?
boB
Im with bob, clamp meters are weird. Hall effect is magnetic, and theres residuals, and stray fields and stuff. (hint dont use it near any kind of magnet, like a motor).
The clamp meter usually has:
- a plus and minus side marked on the jaws.
- an alignment place to put the cable inside the jaws
- a zero feature. You MUST use this every single reading.
- try to keep other wiring a bit aways.
Do all that and you get faiiiiirly consistnent readings, but the accuracy is usally pretty weak at 1-3% or something. Happy days.