How to commission a PV combiner box

Tools: Insulated screw drivers, 600VDC volt meter, electrical gloves.

Notes: The following steps describe how to commission a solar PV combiner box (CB). This article is not intended for troubleshooting, therefore there is not testing to the ground system.

Your multimeter needs to be rated at 600V minimum and be capable of measuring DC voltages and direct currents (DC). A very common multimeter among solar installers is the Fluke Clamp Meter 337.

1) Disconnect and tag all inverters and system disconnects. Put your electrical gloves on and remove or open the lid of the combiner box. Take few minutes to get familiar with the system. Use the system?s electric diagrams to know total number of combiner boxes, strings per combiner, modules per strings and what kind of inverter or equipment you will find downstream.


Working with a PV Combiner Box
Working with a PV Combiner Box

2) Estimate the values you are expecting to find for the voltage and current. Use the module manufacturer spec sheets to know Voc, Isc, Vmp, Imp for the PV modules. You can also find this information in the back of the modules. You can use the following quick formulas:

For string voltage: Vmp x module per string x temp coefficient

For string current: Imp x irradiation factor x performance factor

3) At some point you already took your gloves off, put them back on and open all the circuits in the combiner box by opening the fuse holders. Perform a visual inspection of the fuses; make sure they look OK and that the rating of the fuse corresponds to the rating specified by the module manufacturer.

4) Check polarity for all the strings by measuring the voltage differential of the positive end (fused) to ground. Also make sure that all strings are properly labeled indicating polarity and string number.

5) With your inverter still off and all DC circuits open, measure the voltage between the positive and the negative ends at the combiner box. This is perhaps your best indicator to know that all strings are wired correctly. Make sure that the voltage readings you get are: similar to what you estimated in step 2, close to each other within 2% difference, of the same polarity. If the irradiation is consistent the Voc readings should be very similar for all strings, 2% at the most. For example, is most readings are around 350VDC, none of the string should be above or below that values by more than 7 volts.

6) Check torque for all the connections. Close all the fused circuits, turn on your disconnects and restart you inverters. Wait 5 minutes for the PV system to start operation. Measure the current of every string by clamping your meter to the positive lead cable. Double check polarity. If you have an irradiance meter, use the formula in step 2 and see how close you are from the estimated value. Similar to Voc, the current should not fluctuate much between strings in a day with a clear sky. In overcast days this step can get a little tricky but always make sure that all strings are producing a reasonable amount of energy.

Nicely wired combiner box
Wired solar combiner box

7) Last measure the current going from your combiner box to the inverter (or disconnects) downstream. This value has to be equivalent the sum of all current reading you gathered in step 6. Document all reading and findings, date you report, take some pictures of the condition you left the combiner box. Your solar array has been commissioned!