MCB Tripping Issues During PC Startup – Inrush Current Problem?

S

sareehy

New User
Location
Irbid
Occupation
Electrical technician
Hello All,

I’m facing an issue with MCBs tripping during computer startup and would appreciate your technical insights. Here are the details:

Case 1:
We have a sub-circuit MCB C32 supplying power to 8 desktop PCs. Occasionally, the breaker trips immediately after startup.

Case 2:
At another location, 30 PCs are connected across two sub-circuit MCBs (C32 each), with the main distribution panel protected by an MCB C100. Here, the main breaker trips immediately after startup, sometimes with a visible spark. The sub-circuit breakers also trip occasionally.

Observations:
Current measurements show a significant spike during startup before tripping occurs.
When PCs are completely disconnected from power (by unplugging their power cords ), no tripping issues occur.

Questions:
1. Is this behavior likely caused by excessive inrush current during PC startup?
2. What are the recommended solutions to prevent these nuisance trips?

Best regards,
 
PC power supplies will have a significant inrush current when first powered up as the capacitors in the SMPS charge up. When I was doing power supplies, I once saw a 400W PSU draw 29A on initial turn-on. In good PSU designs, they take steps to limit this inrush current, often using NTC resistors or current limiting resistors and a bypass relay. If these were working fine for a while and started exhibiting this behavior, it might be that the inrush protection circuits have broken down over time or were damaged by a surge or something. That might spell trouble for the PSUs as well, because the capacitors will damage themselves. If this is a first time situation, you may need to change the way you power them, i.e. fewer PSUs on each breaker, or maybe breakers with a trip curve more tolerant of high inrush, like a D curve.
 
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