I was replacing a power board with a new (plastic insulated) one, which resulted in a trip of the 20A 30mA RCBO on the circuit instantly upon turning on one of the board outlets. The built-in 10A breaker on the power board did not trip.
Since all loads previously worked fine on the old power board, I assumed something was wrong with the new power board and begun to investigate.
I found that inside the power board, both active and neutral is routed through a PCB, and a trace ~1mm wide on the active side of the outlet had vapourised and covered the area in a thin layer of black dust.
A 1mm wide trace is clearly too narrow to carry the rated 10A of current of this board, so it doesn't surprise me that it blew up. (The rest of the trace was 5mm wide, but the 1mm part was next to a slot. So I assume it was a misalignment during manufacturing, where the slot cut into the trace more than intended)
However, how did it trip the RCBO?
As far as I know, the RCBO should only trip instantly on either ~30mA of leakage current, or >100A of overcurrent.
So I considered the options:
- Ground fault in outlet: The trace that blew up was nowhere near the ground line (~10cm of separation). I also checked the continuity around the blown-up trace (active) and ground line, and it was definitely open. Neutral to ground was also open.
- Short in outlet: Area around blown up trace (active) to neutral was also open. And the 10A breaker on the board did not trip.
- Fault in load: The load that was connected to it previously worked on the old power board, and continues to work in a different outlet.
So right now, I'm guessing that as the 1mm trace got vapourised, it greatly lowered the ionisation potential of the air inside the board, and formed a conductive path between active and ground (via arc or corona discharge), causing a ground fault and tripping the RCBO. After the dust settled, the conductive path disappeared.
Does that even sound possible? Is there a less outlandish explanation of why the RCBO tripped?
EDIT: I've now repaired the blown-up trace with a thick solder bridge, and now all outlets on the board work as normal. Which is more evidence that the trace vapourisation was what caused the trip
Actual sequence of events for context:
- Switched off all outlets on both old and new power boards, and at the wall
- Replaced the old power board with the new, and plugged everything in
- Switched on the outlet at the wall, and on the board slowly one-by-one (downstream devices automatically start drawing power)
- RCBO trips instantly upon switching on the last and highest downstream load (~1A continuous; unknown inrush) outlet
- Leaving outlets on the on position, I try to reset the RCBO, but it continues to trip
- Switched off the "offending" outlet on the power board and reset the RCBO, and it now stays on
- Try switching on the "offending" outlet again, and the RCBO doesn't trip, but there appears to be no power coming from the port anymore (vapourised trace is no longer conductive now)
- Moved the "offending" load to a different outlet on the board, and switch it on, and this time everything works with no problems
- Disconnected everything, opened up the power board, and found the vapourised trace
- Cleaned the black dust off, reverse engineered the board, and started probing around