Version 1.0, Nov.
2019
String Testing
When String Testing Is Necessary
Pairing and production failure can occur when a string has a bad internal connection or where the string lands on the
inverter. If at least ONE string connected to the inverter is producing AC power then the inverter and DC switch are
NOT the cause of the problem – no strings will produce if the inverter hardware is causing the problem. If you
encounter a situation where at least one string is pairing/working and a second (or third) string fails to pair/produce,
the problem is with the string and will require string testing.
WARNING!
This guide is intended to aid in troubleshooting a SolarEdge installation that is having pairing or
production issues with a string. Persons using this guide should be completely familiar with
SolarEdge systems, their concept of operation, safety features, and all applicable safety procedures
and requirements. Do not attempt any troubleshooting without adequate safety equipment and a
thorough understanding of all procedures.
SafeDC = 1V in Open-air (And Under Load)
SolarEdge power optimizers will output ~1Vdc (0.9 to 1.1Vdc) when connected to a solar panel. Thus, a proper
SafeDC voltage on a string should be approximately 1V per optimizer. If you have 10 optimizers in a string, you
should expect ~10Vdc across the string in open-air.
However, if the string has a bad connection somewhere along the line it is possible for the string to still read 1V per
optimizer in open-air, yet the string will fail to pair or will pair but fail to produce.
Bad connections are the most frequent cause for single strings failing to pair or produce.
Remember to have the IO toggle switch on ‘0/OFF’ during testing. Always drain out any boosted DC voltage and turn
off the DC Disconnect switch prior to changing string wiring.
1. Verify you are getting 1V per optimizer in open-air, document the measurement. If your string voltage is
abnormal or fluctuating in open air, check all optimizer DC outputs and all connections in the string to verify
all connections are solid and all optimizers are pushing 1Vdc. Reduced voltage, fluctuating voltage, or
voltage charges up then drains down behaviors in open air are a sign of a very loose connection in the
string.
2. If string open-air voltage is solid, connect ONLY the string being tested to the inverter. No other strings
should be connected as they will mask the behavior we are looking for.
3. With only the single string connected, turn the DC switch on. Production IO switch should remain OFF – The
string is now under electrical load.
4. Check the Vdc reported by the inverter – it should be pretty close to what you measured in open air. If the
Vdc seen by the inverter is lower by more than a couple volts, or if the Vdc is fluctuating or charging
up/draining down, it is confirmation of a loose connection – check all the DC connections in that string (and
any junction boxes or terminal blocks making DC connections.
5. Once the string Vdc is approximately what was measured in open air and is holding steady the string should
pair and produce.
String Pairs, Doesn’t Produce
If at least one string is working, the inverter and DC switch are not the cause of the problem. If a single string is pairing but failing to
produce it is also likely to be a loose connection in the string. In some cases, optimizer communication signals can ‘jump the gap’
while the DC voltage is unable to do so and production stalls. If you encounter this issue, proceed with the testing outlined above.
String Testing
