MCB tripping intermittently

[Buck Taylor]

house 3 years old
Tripping MCB intermittently in the last 6 months
Grundfos STN 3.0

I have also posted this on the electricians forum and advised that our water friends could maybe help out.

The mains water supply comes into the kitchen from the street and then goes up to the water tank in the loft via a PRV which I have changed and fitted a non return valve on the hot tap side. However, the MCB for the pump is tripping intermittently when and only when I use the tap in the kitchen which is a mono mixer tap. I have also renewed the tap in the kitchen.
Any help/advice would be appreciated.
Thanks

 

[Mark K]

house 3 years old
Tripping MCB intermittently in the last 6 months
Grundfos STN 3.0

I have also posted this on the electricians forum and advised that our water friends could maybe help out.

The mains water supply comes into the kitchen from the street and then goes up to the water tank in the loft via a PRV which I have changed and fitted a non return valve on the hot tap side. However, the MCB for the pump is tripping intermittently when and only when I use the tap in the kitchen which is a mono mixer tap. I have also renewed the tap in the kitchen.
Any help/advice would be appreciated.
Thanks

Pumps can be a pain when they intermittently trip MCBs.

This is a twin outlet shower/booster 3 bar pump right? It's definitely tripping the MCB and not an RCD? Does the MCB feed only the pump? How old is the MCB? What rating is it, i.e. amps and curve type, it will say B10, B16 or C10 or C16 or something similar on the front.

Some more information on what the pump is used for would be handy. Is it used simply to boost the hot water to the kitchen mono tap to match the mains pressure there, in which case only that tap would cause it to run.

Or does it run other outlets?

If it does, I suspect they are showers and use hot and cold water, whereas the kitchen mono may only use the hot water - that could be the issue, putting a physical resistance on the cold water pump causing the impeller to move and create an earth leakage, which doesn't happen when the pumps on both ends of the motor have flow. It could also cause the motor to simply work harder, hence drawing more current, and.... With the changes you have made you could now have less resistance on the hot side causing a bigger imbalance, or more resistance, increasing the work the pump has to do.

It's also possible the water pressure in the cold "pump" when this is happening could be causing water seepage along the spindle, which finds it's way into the motor.

To eliminate anything other than the pump motor, like a cable touching the hot water pipe somewhere on it's run to the tap which moves slightly when the pipe heats up, turn off the pump locally and check that the MCB doesn't trip.

This is all supposition on my part, as I said more info would help and the result of the first diagnostic, switch off the pump to eliminate any other cause.

 

[Buck Taylor]

Pumps can be a pain when they intermittently trip MCBs.

This is a twin outlet shower/booster 3 bar pump right? It's definitely tripping the MCB and not an RCD? Does the MCB feed only the pump? How old is the MCB? What rating is it, i.e. amps and curve type, it will say B10, B16 or C10 or C16 or something similar on the front.

Some more information on what the pump is used for would be handy. Is it used simply to boost the hot water to the kitchen mono tap to match the mains pressure there, in which case only that tap would cause it to run.

Or does it run other outlets?

If it does, I suspect they are showers and use hot and cold water, whereas the kitchen mono may only use the hot water - that could be the issue, putting a physical resistance on the cold water pump causing the impeller to move and create an earth leakage, which doesn't happen when the pumps on both ends of the motor have flow. It could also cause the motor to simply work harder, hence drawing more current, and.. With the changes you have made you could now have less resistance on the hot side causing a bigger imbalance, or more resistance, increasing the work the pump has to do.

It's also possible the water pressure in the cold "pump" when this is happening could be causing water seepage along the spindle, which finds it's way into the motor.

To eliminate anything other than the pump motor, like a cable touching the hot water pipe somewhere on it's run to the tap which moves slightly when the pipe heats up, turn off the pump locally and check that the MCB doesn't trip.

This is all supposition on my part, as I said more info would help and the result of the first diagnostic, switch off the pump to eliminate any other cause.

Hi Mark,

Thanks for the swift reply.

It is a twin outlet shower booster pump.
It does trip the MCB and I have also changed the MCB type b 6a to a typeC 6 amp MCB as advised on the electrical forum.
The pump kicks in for every outlet in the house (shower and bath upstairs)except just cold water at the kitchen tap as that is fed directly from the street pressure, but when I open the hot side of the mono tap then pump kicks in. (Hope that makes sense)

 

[Buck Taylor]

Pumps can be a pain when they intermittently trip MCBs.

This is a twin outlet shower/booster 3 bar pump right? It's definitely tripping the MCB and not an RCD? Does the MCB feed only the pump? How old is the MCB? What rating is it, i.e. amps and curve type, it will say B10, B16 or C10 or C16 or something similar on the front.

Some more information on what the pump is used for would be handy. Is it used simply to boost the hot water to the kitchen mono tap to match the mains pressure there, in which case only that tap would cause it to run.

Or does it run other outlets?

If it does, I suspect they are showers and use hot and cold water, whereas the kitchen mono may only use the hot water - that could be the issue, putting a physical resistance on the cold water pump causing the impeller to move and create an earth leakage, which doesn't happen when the pumps on both ends of the motor have flow. It could also cause the motor to simply work harder, hence drawing more current, and.. With the changes you have made you could now have less resistance on the hot side causing a bigger imbalance, or more resistance, increasing the work the pump has to do.

It's also possible the water pressure in the cold "pump" when this is happening could be causing water seepage along the spindle, which finds it's way into the motor.

To eliminate anything other than the pump motor, like a cable touching the hot water pipe somewhere on it's run to the tap which moves slightly when the pipe heats up, turn off the pump locally and check that the MCB doesn't trip.

This is all supposition on my part, as I said more info would help and the result of the first diagnostic, switch off the pump to eliminate any other cause.

Sorry the MCB only feeds the pump and it is 3 years old.

 

[Mark K]

Hi Mark,

Thanks for the swift reply.

It is a twin outlet shower booster pump.
It does trip the MCB and I have also changed the MCB type b 6a to a typeC 6 amp MCB as advised on the electrical forum.
The pump kicks in for every outlet in the house (shower and bath upstairs)except just cold water at the kitchen tap as that is fed directly from the street pressure, but when I open the hot side of the mono tap then pump kicks in. (Hope that makes sense)

OK, when you turn the kitchen tap on, does it trip instantly, or after a few seconds?

Would it trip if you ran an upstairs cold tap and turned on the kitchen tap while that was running?

What size cable feeds it and how long is the run?

Do you have local isolating valves either side of the hot and cold sides of the pump?

 

[Buck Taylor]

OK, when you turn the kitchen tap on, does it trip instantly, or after a few seconds?

Would it trip if you ran an upstairs cold tap and turned on the kitchen tap while that was running?

What size cable feeds it and how long is the run?

Do you have local isolating valves either side of the hot and cold sides of the pump?

On the occasions When it trips it hums for a 5-10 seconds then trips.

Not tried the running of another cold tap (yet)
The cable size is 2.5mm and the run is no more that 8 metres.

There is an isolating valve on the inlet for the cold but not the hot.

Thanks again for your help on this.

 

[Chuck]

"When it trips it hums for a 5-10 seconds then trips."

This is a sign that the motor is not turning although current is passing through the windings. Could be a faulty pump, e.g. a bad capacitor.

If not, that's a fairly beefy pump to have feeding a kitchen tap. I suspect that you may need a bypass around the pump. Check the installation instructions very carefully and then, if necessary, consult Grundfos.

 

[Buck Taylor]

That's a fairly beefy pump to have feeding a kitchen tap. I suspect that you may need a bypass around the pump. Check the installation instructions very carefully and then, if necessary, consult Grundfos.

Hi chuck,
It feeds all outlets in the house except the main feed (cold) (kitchen tap)when it comes into the house.

 

[Mark K]

On the occasions When it trips it hums for a 5-10 seconds then trips.

Not tried the running of another cold tap (yet)
The cable size is 2.5mm and the run is no more that 8 metres.

There is an isolating valve on the inlet for the cold but not the hot.

Thanks again for your help on this.

It sounds like when it trips the motor is drawing just over or close to the mcb limit. I assume you mean the mcb hums, not the motor??

I suspect the cold pump is creating work for the motor by pumping against a closed head when you open only the hot at the kitchen tap. Running a cold tap would eliminate that.

Also, isolating the cold side would do the same, if you emptied the pump body of water, but you wouldn't want to run it for long like that.

You need to do some of those checks now and report back before we can go any further.

 

[Chuck]

Hi chuck,
It feeds all outlets in the house except the main feed (cold) (kitchen tap)when it comes into the house.

My reading of your description was that it the MCB only cuts out when the kitchen hot tap is opened on its own. Also, that it was the motor humming prior to the MCB tripping. Did I get these wrong?