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.

 

[telectrix]

pump is a motor. maybe a type C breaker is required to cope with the inrush/start current.

 

[westward10]

Is this a recent occurrence.

 

[Yozza19]

Need to make sure disconnection times alow change to type c

 

[Wilko]

Hi - picking up @telectrix point - Can you advise the make and model of the pump please?

 

[Buck Taylor]

Is this a recent occurrence.

around 6 months, my neighbour also has the same issue!

 

[Buck Taylor]

Hi - picking up @telectrix point - Can you advise the make and model of the pump please?

It's a Grundfos shower pump STN3.0

 

[Buck Taylor]

pump is a motor. maybe a type C breaker is required to cope with the inrush/start current.

Cheers Scouse, I'll buy a type C breaker, I take it they are both physically the same size?

 

[Buck Taylor]

Need to make sure disconnection times alow change to type c

Thanks Yozza, I'll try this, pulling my (what's left of it, hair oot)

 

[Buck Taylor]

pump is a motor. maybe a type C breaker is required to cope with the inrush/start current.

 

[suffolkspark]

Yes as long as you brand match, something that recent should still be the same

 

[Buck Taylor]

Thanks for your advice.

 

[Wilko]

It's a Grundfos shower pump STN3.0

Hi - reading the manufacturer's instructions it says to be connected via a fused connection unit with a 5A fuse (me - this would normally be a spur off a power circuit with its own 16A to 32A mcb) What's the story with using 6A mcb?

 

[Marvo]

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

 

[marconi]

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

 

[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?