MCB tripping intermittently

[Buck Taylor]

• new build house 3 years old

• Cold water supply comes into the house at the kitchen

When turning the mixer tap on in the kitchen it sometimes trips the water pump, it does not trip the pump from any other point in the house either mixer or single tap.

I have changed the pressure reducing valve, renewed the mixer tap and fitted a non return valve on the hot outlet under the kitchen sink and replaced the 6a MCB.

Any help advice greatly appreciated.

 

[Buck Taylor]

Buck Taylor RE # 1: Before you modify the electrical supply to the pump you should confirm first that the Grundfos pump is plumbed correctly. For starter, it must not be plumbed directly off the cold water mains - it must have a feed from a cold water storage tank located above the pump.

See Section 3 Installation of:

Grundfos https://showerpumpselector.co.uk/pdfs/installation-advice.pdf

Hi, thanks- yeah their is a cold water tank in the loft space. I have replaced the 6 amp type B With a 6amp type C MCB and it is still tripping?

 

[telectrix]

you could always advise the customer to bath in the Clyde..

 

[marconi]

I need some more information please:

a. What does it say on the pump rating plate for running current and for power?

b. What is the size and installation method of the cable feed to the motor from the CU?

The manufacturer states a 5A fused spur and thus BS1361 fuse protects the pump. I suspect - from some quick comparison calculations between a 5A BS1361 and a type C 6A mcb - that the product of the pump motor's short (less than 1s) start time and square of the high starting current(5-10 In) exceeds the no-trip Isquaredt of the 6A mcb by some margin but not the pre-arcing Isquaredt for the 5A BS1361 fuse (which is 44A2s). And that the fuse provides some current limiting which helps matters.

What I would do is a quick experiment: at the CU see what happens if you swap the 6a mcb for a 5A HRC or a BS1361 fuse and whether then the pump starts without problems.

 

[Wilko]

Hi, thanks- yeah their is a cold water tank in the loft space. I have replaced the 6 amp type B With a 6amp type C MCB and it is still tripping?

Hi - why is it on a 6A MCB ? Commonly they would be on circuit with at least 16A MCB, via a 5A fuse as per @marconi.
Then if it blows the manufacturer specified 5A fuse we are headed in a certain direction ...

 

[westward10]

Everything was fine until six months ago?

 

[Buck Taylor]

As above, rate the MCB according the the circuit wiring size and fit a FCU at the pump with a suitably sized fuse..

Yeah FCU is fitted adjacent to pump. It has worked ok for 2.5 years and now playing up.

 

[Buck Taylor]

Everything was fine until six months ago?

Yeah pretty much! That's what it is like needle in haystack !!

 

[Buck Taylor]

Buck Taylor RE # 1: Before you modify the electrical supply to the pump you should confirm first that the Grundfos pump is plumbed correctly. For starter, it must not be plumbed directly off the cold water mains - it must have a feed from a cold water storage tank located above the pump.

See Section 3 Installation of:

Grundfos https://showerpumpselector.co.uk/pdfs/installation-advice.pdf

Hi, no it's fed directly and has worked ok for last 2.5 years!

 

[telectrix]

could be a winding failure in the pump itself, drawing excessive current.

 

[Wilko]

Yeah FCU is fitted adjacent to pump. It has worked ok for 2.5 years and now playing up.

So there's a 5A fuse and it doesn't blow?

 

[Silly Sausage]

Hi, no it's fed directly and has worked ok for last 2.5 years!

To be clear now, you're saying the cold water feed to the pump IS direct off the mains supply?
If so, and next door is the same with the same fault, that's got to be the source of the problem.
Maybe there has been a change in the mains water supply pressure over the last six months.

All pure speculation of course!

PS, have you tried asking our watery friends on their forum?
Gas Engineer & Plumbing Advice Forum - https://www.ukplumbersforums.co.uk/

 

[westward10]

May want to ask a few other neighbours.

 

[DPG]

I thought you mentioned it was fed from a water tank?

 

[marconi]

I have contacted Grundfos for details of the STN3.0. It is powered by an induction motor and on the specification plate it states its power is 850W and the running current is 4A.

 

[marconi]

it worked fine for 2 and half years - Just wondering - does your kitchen mixer tap have a strainer and if so is it in need of unblocking or replacement? If you remove it does it make any difference to the pump trip problem?