Internal DC Short: In the inverter head, or the wirebox?

When open-circuit DC string voltage is measured and found to be within the expected range and polarity, but drops to 0 Vdc - 10 Vdc when the DC switch is closed, it indicates a DC short circuit internal to the inverter.  This can be confirmed by checking DC string current with a clamp meter.  Voltage near 0 Vdc coupled with a high DC amperage measurement when the DC switch is closed confirms the presence of an internal short circuit.  

If the inverter has an integrated wire box and has an internal short circuit, the entire inverter will need to be replaced.  If the inverter has a separate wirebox, it is useful to determine if the short is internal to the inverter head, or if it is internal to the wirebox.


Short circuits are more likely to be internal to the inverter head when a short is detected on only one of the MPPTs.  Inverters will often be able to run when only one MPPT is shorted, but will appear to have a string down. If a short is detected on all of the MPPTs, it is more likely that the short is located within the wirebox, and in particular, the switching mechanism.  If all the MPPTs are shorted, the inverter most likely will not start up at all.


After confirming the presence of an internal short, check to see if it is located in the wirebox or the head by performing the following steps:

  1. With the wirebox closed, turn off the AC breaker supplying the inverter with grid voltage.  Then, turn off the DC switch.
  2. Isolate the DC voltages from the inverter head by opening all the fuse holders. (Always use DC clamp to verify no current is present before opening fuseholders!)
  3. Wait 3-5 minutes to allow residual DC voltage in the inverter top to dissipate.
  4. Unbolt the inverter head from the wirebox, and have a second person assist in removing it.  Be sure to keep the contacts on the bottom of the inverter head clean when choosing a place to set it down.
  5. With the wirebox isolated from the inverter head, close the DC fuse holders, and measure DC voltage on each MPPT.  Then, close the DC switch, and measure DC voltage on each MPPT again.
  6. If the expected DC voltage is measured between positive and negative MPPT terminals when the switch is open, but drops to 0 Vdc - 10 Vdc after the switch is closed, then the short is limited to the wirebox.  Only the wirebox needs to be replaced.  
  7. If the DC voltage measures the same whether the switch is open or closed, then the inverter head needs to be replaced.
  8. While the inverter head and wirebox are separated, visually inspect the contacts on each for signs of damage. Damage to these components is uncommon, but if present, could require the replacement of both inverter head and wirebox.
J
John is the author of this solution article.

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