Puzzling Volt Drop When Pump Connected

Revved Up Sparky · Jul 26, 2010

I was fault finding last Friday at a house with a fully pumped hot water system. A brand new pump had been fitted but it would'nt work.

When I put my voltmeter across the L & N feeding the pump I recorded a voltage of 174V AC. L-E was 240V. However when I disconnected the L from the pump the voltage between L-N went back up to a healthy 240V AC.

Any ideas on what could be causing this volt drop ?. I was only there for a few minutes and did'nt check to see if the motor would turn manually. Would a jammed motor produce such a volt drop ?.

I will be going back to this job in the next day or two so I would really appreciate any advice/ pointers.

Thanks in advance.

kme · Jul 26, 2010

Looks / sounds like a faulty N connection somewhere - maybe through the return of a valve? Looks like a N supply until you load it, then you`re actually putting 2 loads in series. I`d check for the pump supplies being correct - usually the pump is fed from the same point as boiler demand.....

HTH

KME

Evans Electric · Jul 26, 2010

As KME says, that N is not connected properly , I would think, somewhere downstream . Always check the pump with a plug top and flex. You can usually spin those pumps by removing the big silver screw head , screwdriver slot under there is the rotor .

Revved Up Sparky · Jul 26, 2010

Thanks guys I really appreciate it - wise words - and good idea putting the pump on a plug for elimination.... I'm a fairly experienced spark but just did'nt think to do that on Friday !. The bad neutral theory sounds very plausible. I will let you know what I find later this week.

Andy™ · Jul 26, 2010

definately sounds like dodgy neutral

Revved Up Sparky · Jul 29, 2010

I just wanted to let KME, EE and Andy know that I went back to that job today and put a plug & flex on the pump - it ran fine so I re-checked the neutral and sure enough it was not functioning as a neutral- you were all spot on !.

This afternoon I spent several hours up a ladder installing PVC conduit on the outside wall from the attic (where the DHW equipment & controls were) down to the boiler house before installing a new switchwire and neutral from the timeclock (in the BH). All is well with pump running and 240V across the L&N terminals.