Weird MCB Query

[jb567]

Earlier today I turned off the power to consumer unit as I wanted to change a light switch. After changing the switch I reactivated the consumer unit main 100amp breaker, at which point the MCB for the Ring Final at the back of the house, on a completely different circuit activated and turned off. The MCB was completely unconnected with the light switch I changed and reactivated no issues whatsoever. I have an insulation and continunity tester from a former life, albeit it is slightly old now, which I have used to test the circuit with good readings across the board. There is no regular tripping in the house and in the 6 years ive lived here never had a MCB tripping episode. There's been no recent rain, albeit I'd expect that the trip the RCD not the MCB and no recent DIY that would cause an issue.

Is there a simple explanation as to why the MCB activated when power was reapplied when I reactivated the main 100A breaker?

 

[flameport]

A spike of high current due to whatever was connected to it at the time all being switched on at once.
Less likely is magnetic effects from the adjacent MCBs.

 

[jb567]

A spike of high current due to whatever was connected to it at the time all being switched on at once.
Less likely is magnetic effects from the adjacent MCBs.

I thought that would be an explanation, there isn't that many high current devices on the circuit, but I guess a culmination of every turning on all at once could add up.

 

[AdrianUK]

I thought that would be an explanation, there isn't that many high current devices on the circuit, but I guess a culmination of every turning on all at once could add up.

Not necessarily high current devices. Alot of modern consumer electronics use switch mode power supplies (SMPSUs) as the device to turn the mains into low voltage DC. These have a rectifier & a large capacitor on the front end. Each of these can easily draw several tens of amps for a few cycles while the capacitor charges up.

A few of these on a circuit (TV, PC, monitors, SKY box etc) & its easy to trip a B32A MCB. Since the caps now have some charge in them, when you reset the MCB, this time it holds.