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?

 

[Buck Taylor]

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?

No your correct, the motor/pump hums and the MCB only trips (intermittently) when opening the tap in the kitchen.

 

[Mark K]

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?

My understanding also Chuck, is that it only trips when (only?) the kitchen tap is used, which is only on the hot side of the pump.

He hasn't said if it's the mcb or the motor that hums. As you say, if it's the motor then that could be faulty, but I'm barking up the "that would be the same whichever outlet was opened" tree.

I wonder if the other outlets in the house are mixers, so everything except the kitchen causes flow on the cold and hot.

Need more info from the OP I feel.

 

[Buck Taylor]

My understanding also Chuck, is that it only trips when (only?) the kitchen tap is used, which is only on the hot side of the pump.

He hasn't said if it's the mcb or the motor that hums. As you say, if it's the motor then that could be faulty, but I'm barking up the "that would be the same whichever outlet was opened" tree.

I wonder if the other outlets in the house are mixers, so everything except the kitchen causes flow on the cold and hot.

Need more info from the OP I feel.

Hi Mark,
It is the pump that hums as the consumer unit is in the garage and I would not be able to hear that whilst in the house. I have a few other mixer taps in the house and single taps and tried many times to trip the pump and been unsuccessful.
Your diagnosis is spot on Mark. it is something to do with the mains pressure / hot tap. I have also bled the system to ensure their are no air pockets working from the bottom of the house up.

 

[Mark K]

Hi Mark,
It is the pump that hums as the consumer unit is in the garage and I would not be able to hear that whilst in the house. I have a few other mixer taps in the house and single taps and tried many times to trip the pump and been unsuccessful.
Your diagnosis is spot on Mark. it is something to do with the mains pressure / hot tap. I have also bled the system to ensure their are no air pockets working from the bottom of the house up.

OK. Chuck was right about the humming and I was wrong. Without seeing it, I am going to say the resistance (water) at the kitchen tap is higher than any other tap. This is causing the motor to work harder and draw more current.

What I would want to hear, this will take teamwork, is, while the pump is humming, another tap is opened and see if the motor stops humming and starts pumping.

 

[Buck Taylor]

OK. Chuck was right about the humming and I was wrong. Without seeing it, I am going to say the resistance (water) at the kitchen tap is higher than any other tap. This is causing the motor to work harder and draw more current.

What I would want to hear, this will take teamwork, is, while the pump is humming, another tap is opened and see if the motor stops humming and starts pumping.

Yeah understand, I'll give it a go ( in house myself at moment) have a toilet downstairs I can get to tap, if I run fast !!!
If not I can try when the missus comes home.
Thanks guys really appreciate the help.

 

[Chuck]

I'm barking up the "that would be the same whichever outlet was opened" tree.

I suspect it's a motor problem but it's borderline-symptomatic at the moment and the kitchen tap, for the reasons you listed, is the worst case. I.e. starting against near-closed outlets.

There's another clue buried in the symptoms, which is that the system has been in for 3 years but this started as a problem only 6 months ago. Of course, it's difficult to decide whether the pump is dying of natural causes or whether something about the installation is overstressing it.

My next move would be a forensic comparison of the installation with the Grundfos instructions. IIRC those pumps have a minimum displacement rate of 5 l/min, for example. Grundfos also have a faultfinding page:

Technical Troubleshooter - Grundfos

and a contact page for 'faults not covered', which is the case here:

Contact - Grundfos

 

[Buck Taylor]

I suspect it's a motor problem but it's borderline-symptomatic at the moment and the kitchen tap, for the reasons you listed, is the worst case. I.e. starting against near-closed outlets.

There's another clue buried in the symptoms, which is that the system has been in for 3 years but this started as a problem only 6 months ago. Of course, it's difficult to decide whether the pump is dying of natural causes or whether something about the installation is overstressing it.

My next move would be a forensic comparison of the installation with the Grundfos instructions. IIRC those pumps have a minimum displacement of 5 l/min. Grundfos also have a faultfinding page:

Technical Troubleshooter - Grundfos

and a contact page for 'faults not covered', which is the case here:

Contact - Grundfos

Thanks Chuck I've been in contact with Grundfos numerous times. The advise they gave was fitting a non return valve on the hot inlet to prevent any back pressure from the mains cold feed. It done diddly squat. They also give 5 year warranty but only 2 years for domestic customers? What's that all about.

 

[Mark K]

Thanks Chuck I've been in contact with Grundfos numerous times. The advise they gave was fitting a non return valve on the hot inlet to prevent any back pressure from the mains cold feed. It done diddly squat. They also give 5 year warranty but only 2 years for domestic customers? What's that all about.

I think that's your answer...they are worried about back pressure from the mains cold water onto the hot. What that would do is restrict flow. But, so would having no opening on the cold tap, or a very restrictive tower tap on the hot.

As Chuck says, this has been going on for 2.5 years, giving the motor and it's capacitor a hard time, and now it's showing up.

I suspect they would say it's an incorrect installation causing flow below the minimum if you went for a warranty claim.

 

[Buck Taylor]

Yeah understand, I'll give it a go ( in house myself at moment) have a toilet downstairs I can get to tap, if I run fast !!!
If not I can try when the missus comes home.
Thanks guys really appreciate the help.

OK. Chuck was right about the humming and I was wrong. Without seeing it, I am going to say the resistance (water) at the kitchen tap is higher than any other tap. This is causing the motor to work harder and draw more current.

What I would want to hear, this will take teamwork, is, while the pump is humming, another tap is opened and see if the motor stops humming and starts pumping.

Hi Mark/Chuck,
With the pump in the hum state I opened up another tap immediately and the MCB still tripped.

 

[Mark K]

Hi Mark/Chuck,
With the pump in the hum state I opened up another tap immediately and the MCB still tripped.

Definitely points to a failed/failing motor then.

What happens if you reset it and open a different tap?

 

[Buck Taylor]

Would fitting a PRV under the kitchen sink and setting it at 3.0 bar direct to the cold tap help?
There is also a PRV fitted going up to the header tank in the loft!

 

[Chuck]

Hi Mark/Chuck,
With the pump in the hum state I opened up another tap immediately and the MCB still tripped.

In your position, I'd try replacing motor's starting capacitor. You need to get the right type and size. Motor repair companies are relatively common so, if you're not sure what you're doing, check the Yellow Pages. They'll also be able to check the windings and bearings for you.

I now suspect that when the high demand outlets are used they are actually using the standing pressure to turn the impellor blades, which helps start the motor.

 

[Chuck]

Would fitting a PRV under the kitchen sink and setting it at 3.0 bar direct to the cold tap help?

I very much doubt it.

 

[Buck Taylor]

Definitely points to a failed/failing motor then.

What happens if you reset it and open a different tap?

Hi Mark,
It trips when either hot or cold tap is opened

 

[Mark K]

Hi Mark,
It trips when either hot or cold tap is opened

Yes, so once it's stalled (buzzing) it's going to trip, presuming you mean while it's buzzing.

Does it still work when other taps are opened from an off state?

 

[Buck Taylor]

Yes, so once it's stalled (buzzing) it's going to trip, presuming you mean while it's buzzing.

Does it still work when other taps are opened from an off state?

Yeah when buzzing and opening up another tap it trips the MCB.
Not sure what you mean about opening another tap from an off state?
The MCB has never tripped from opening any other tap in the house!

 

[Mark K]

Yeah when buzzing and opening up another tap it trips the MCB.
Not sure what you mean about opening another tap from an off state?
The MCB has never tripped from opening any other tap in the house!

I mean what you said..no other tap opening trips it.

As Chuck said it sounds like a motor and/or capacitor failing. The characteristics of the kitchen tap cause a higher current draw, which causes the breakdown and the motor stalls and won't start. Opening other taps doesn't cause the initial current draw, and hence heat, so the motor gets going.

I bet you can open the kitchen tap no problem if another tap is already running?

 

[Buck Taylor]

I mean what you said..no other tap opening trips it.

As Chuck said it sounds like a motor and/or capacitor failing. The characteristics of the kitchen tap cause a higher current draw, which causes the breakdown and the motor stalls and won't start. Opening other taps doesn't cause the initial current draw, and hence heat, so the motor gets going.

I bet you can open the kitchen tap no problem if another tap is already running?

Yeah opening the kitchen tap whilst the pump is running does not trip the MCB. Looked on
showerdoc.com for a replacement starting capacitor and it is sold out!!! Oh the joys I'll look for something compatible.

 

[Buck Taylor]

I mean what you said..no other tap opening trips it.

As Chuck said it sounds like a motor and/or capacitor failing. The characteristics of the kitchen tap cause a higher current draw, which causes the breakdown and the motor stalls and won't start. Opening other taps doesn't cause the initial current draw, and hence heat, so the motor gets going.

I bet you can open the kitchen tap no problem if another tap is already running?

Good morning gents,
The first time this has happened..... The tap in the downstairs toilet has now tripped the MCB, so does that now point more to the start capacitor?

 

[marvoni]

Do you have access to a clamp meter?



If you can beg, borrow or steal one for a few hours it will give you a definative answer.

It's a non-contact tester that measures current but you need to isolate the power whilst you clamp it around either the live wire or a neutral wire of the cable that supplies the pump.

If at any time the current being drawn by the pump exceeds the manufacturers stated maximum current you should replace the pump.

You could try replacing the capacitor but it's a long shot. Capacitor damage is visible in the majority of cases manifesting as a swolen or cracked casing or as burned terminations or other signs or arcing / overheating. Also, if the capacitor is faulty there's a high liklihood it was damaged due to another problem within the pump that caused high peak current or prolonged marginal over-current through it such as mechanical wear, bearing failure or scale build-up so it's more often than not a symptom rather than a root cause.

 

[Buck Taylor]

Gents, many thanks for all your input regarding the MCB tripping intermittently. I m sure to have rectified the cause, which does not make sense.

The inlet strainer on the hot side was very badly blocked with debris, so not sure why the pump didn't trip from all other outlets?
Confused??? Me too, happy days!
Thanks again