Background: South Carolina house built in 2009, assume all electrical systems, main panel and circuit breakers new at that time. I moved into the house in May having transferred from the UK. I brought some electrical equipment with me from the UK, and have been using step-up transformers (SUT) to power the UK items as well as purchasing some new US ones.

About 2-3 weeks after moving in one circuit started tripping, and eventually would not re-set. It fed a secondary bedroom (being used as an office including using a 500W SUT) and adjacent bathroom. It tripped even after unplugging everything, re-setting, and trying one lamp (running 3 x 8W 120V LED bulbs). I was working away and my wife called an electrician. We hoped that the fault might be covered by Homebuyers insurance!

He diagnosed that the SU transformer had caused instability (so not covered by insurance as it's a condition we introduced), that the circuit breakers were sensitive, and that once a breaker starts tripping it's likely to keep tripping. He changed it for one that was 'less-sensitive'.

Since then several other circuits (around 5 different ones) have tripped, some where SUTs are in use, and some not. They often just trip, and don't seem linked to introducing a new load, and normally have fairly low load items plugged in (eg wifi router, LED TV, LED lamp). Some have only tripped once, others more often. Some trip a couple of times (within 5-10 minutes while trying to reset) before staying re-set, others stay reset after one trip.

So far all have re-set and not constantly tripped until last night around midnight - the circuit had a 1000W SUT plugged in and switched on, but all attached appliances were switched off and no new load was introduced.

The breaker will not re-set, even after unplugging the SUT - it trips instantly.

Questions:
- is the link to instability when using SUTs valid?
- if yes could the instability 'transfer' to other circuits where there is no SUT?
- once a breaker has started tripping, will it keep doing it and need replacing?
- is any of this 'normal' in the US? My experience in the UK is that a breaker will rightly trip if my wife boils the kettle and plugs an iron in while the tumble drier is running (!) but otherwise 'new' systems are pretty stable. This seems to be happening almost randomly and when there is no new load being introduced.

Thanks

Edit - some additional thoughts on factors which I have introduced which might be useful (assuming this wasn't happening with previous owners):
- I have added surge protectors, either directly or after SUT to some of the circuits which have tripped.
- I am using a pair of powerline adaptors for wired ethernet which crosses two of the circuits which have tripped (the original secondary bedroom/office problem circuit and a downstairs closet which is on the same circuit as the living room on which is an LED TV, Apple TV and a sounds system all through a surge protector).