Electric Boiler tripping MCB

[krissiboy]

Hi all.

Still a bit new to this so here goes.

Just been called out to an electric central heating boiler. Customer says its been tripping the 40A MCB for a couple of months.- Not often but customer is a landlord and he doesn't want to lose his tenant over this.

Its fed with 10mm t+e cable via a 45a DP sw and is protected by a 30mA RCD. The rating of the boiler is 9kw. I tested the voltage a few times at the heater which came out to be between 236 and 240volts. So doin a bit of maths the current should be at worst 38.14Amps.

Using a clamp meter I found the current to be 40.4Amps. I suppose the circuit board would use a little power too hence the higher current.

Would this very small overcurrent be enough to trip the MCB albeit over a bit of time? I thought about trying a 45a MCB type B but they dont seem to make them for Hager Boards and I'm loathed to stick in a 50A.

So could I put in a 45Amp MCB of another make. Are there any other makes that are compatible with Hager?

Thanks in advance.

 

[Guitarist]

40.4 amps will not trip the 40 amp MCB. However, occasionally an MCB will trip when a high load (close to the MCB rating) switches off due to some switches drawing an arc across the contacts. What does the manual state is required?

 

[Top Cat]

When you say an electric boiler are you talking about an electricair hot air system or a wet system with 9 Kw element?

 

[krissiboy]

Manual states a 40amp MCB. The total wattage is actually 9.15kW but that still comes out at 38.77 Amps..

The 45 amp DP sw is always on for the boiler - the control side (pump and timer) is fed from another circuit.

 

[krissiboy]

When you say an electric boiler are you talking about an electricair hot air system or a wet system with 9 Kw element?

A wet sytem. looks like it has 3 seperate elements. I tested the resistance out of curiosity and they are all roughly the same so didn't think there was out wrong with them

 

[Top Cat]

When you say an electric boiler are you talking about an electricair hot air system or a wet system with 9 Kw element?

Sorry, must be in invisible mode. Let you sort it out amongst yourself's! Lol

 

[resettleddave]

40.4 amps will not trip the 40 amp MCB. However, occasionally an MCB will trip when a high load (close to the MCB rating) switches off due to some switches drawing an arc across the contacts. What does the manual state is required?

Guitarist

For my interest and info, do you think changing the MCB to a type C or D would help with this?

 

[Marvo]

I don't think a slower curve MCB will help with it being a linear load. I would IR test the elements and check all terminations are correctly made and tight. If you're circuit is running close to the rating of the MCB then even a slight resistance on a termination could cause enough heat to cause it to trip on thermal operation.

 

[Deleted member 9648]

After the above^^^ tests, if nothing shows up it could simply be the MCB giving up,not unknown for constantly loaded breakers.Although I doubt an IR test will confirm much,unless the RCD is faulty it is unlikely to be an earth fault.

 

[Edd]

Im asuming the tripping happens after its been running a while?
If thats the case then its most likely tripping on thermal overload. (MCB slowly heating up) Run the thing for 10 minuits and then feel the mcb it shoukd be cold.
People who say their 9.5-10 kw shower runs fine on a 40 A dont suffer from this as its only running for 5 minuits, these electric boilers can run for long period's of time espesially when the water systems cold.

if it trips out randomly when first loaded then what ive said above is not your problem, if all IR tests result in a good reading then change the mcb.

 

[Guitarist]

I don't think a slower curve MCB will help with it being a linear load. I would IR test the elements and check all terminations are correctly made and tight. If you're circuit is running close to the rating of the MCB then even a slight resistance on a termination could cause enough heat to cause it to trip on thermal operation.

Agree mainly with this. The only thing in favour of a type C would be if it is a current spike when the contactor opens. I would try a new type B first, just in case it is faulty (after checking all terminations). Also, have a look at the contactor and make sure there is no sign of burning. Manufacturers have a habit of fitting the smallest rated components nowadays and many fail prematurely.

 

[jumpin jax]

Stick a 50 a in the board feeding a standalone 2 way consumer with a 45 a in it feeding boiler, this should then do what you want in a round about way.

 

[Guitarist]

He needs to comply with manufacturers instructions which state a 40A MCB.

 

[jumpin jax]

Good point Guitarist, but the clamp meter is showing more amps than manual states due to voltage. The boiler is also a resistive load so should not go into overload current any more than it is now, and if it does then there is a boiler fault.

 

[drew35]

These electric boilers are pretty poxy things with most of the manufacturers going out of business so back up is pretty non existent. There is usually a screw somewhere which will allow you to change the load, it allows you to set it anywhere from 8kw up to 10kw. Sometimes the screw is poking through the cover, when I say screw I mean a little plastic slotted screw, so if you think its load you can back it down to the minimum.
There are usually more than three elements and if memory serves they should each be about 19ohms, or possible 9, or there's a nine in it, its been a while sorry.
Usually they have a cut out that detects internal faults, it usually brings on a flashing light on the front panel.
I have had problems with loose connections, burning at the connections, to the elements I mean. And when I have managed to contact the technical help lines with the manufacturer they haven't seemed to know any more then me.