Electramate 2000 9kW Tripping MCB

Hi folks.

My girlfriend has an Electramate 2000 9kW which is intermittently tripping it’s 40A type B MCB that supplies the boiler element(s).

I believe that this main MCB for the boiler should be 45A but this boiler has obviously been fitted for many years without issue.

I have already replaced one of the 25A fuses that supplies the dual elements of the boiler as I found it had blown.

I found that the incoming voltage was a bit high at 248V so originally suspected that both elements were pulling >20A which was causing the 40A MCB to trip as expected.

However, I have tried running the boiler with only one of the 25A fuses fitted (have tried each side individually) but again the 40A MCB trips.

At the moment the boiler is online (with one element only) and is beginning to heat the system. However, it is intermittently tripping the 40A MCB (regardless of which 25A fuse is inserted). Also, when I try to insert both 25A fuses, the 40A MCB trips within a couple of seconds.

I’m not an electrician to trade so I might be missing something obvious, but struggling to see how a single element is tripping the 40A MCB, especially seeing as it is heating up. Some sort of intermittent fault maybe?

Any advice would be greatly appreciated, cheers.

Almost certainly a failing heater element. Immersion elements can fail by the outer copper sheath splitting or cracking and then water creeps in and begins to saturate the powder insulation inside, which holds the element wire itself in the centre. The result is both RCDs tripping and fused blowing.

You can prove it by disconnecting one then the other element and seeing if the tripping stops. A new heater element cartridge will be needed, or "flow boiler" as Gledhill calls it. A really common failure on the Electramate.

Cheers for the reply Mike. I have tried each of the elements individually and they both trip the MCB (there’s not a specific period of time for each one, it’s fairly random). However, if you’re saying it’s a gradual failing of the element the that maybe explains why the fault isn’t totally repeatable every time.

Either way that gives me a bit of confidence to go ahead and order a spare. Am I right in saying there’s an isolation valve either side of the flow boiler? Is the changeout relatively straightforward?

Also, do you think it’s worth upgrading that MCB from a 40A to a 45A, I see the full load current is potentially 41.3A when both elements are going full tilt?

Thought I'd reply incase anyone else comes across this issue.

I replaced the boiler, but this only allowed me to operate with one side of the element on at a time. Trying to run with both 25A element fuses fitted at the same time tripped the 40A MCB.

Replaced the 40A MCB with a like-for-like 40A type B MCB and this remedied the issue.

Looks like it was a combination of faulty element and faulty MCB.