Electrical Issue - Sanity Check Required

[SimonP85]

We've recently had some issues with power on board. Some things were not switching on from the main panel, for example the VHF radio would not turn on and just made a clicking/popping sound and the associated panel "on" lamp was half lit. Checked all connections for the VHF and all seemed ok and the issue seemed to fix itself.

Then last weekend I went to run the electric bilge pump and on pressing the switch various lamps on the panel illuminated (including the VHF) but the pump did not run. This led me to think the pump wiring was causing the issue so checked this over yesterday.

Wiring to the pump all seems ok, 12V at the pump when it should be. Disconnected the pump and the VHF is now working ok and pressing the bilge pump switch does not illuminate random lights on the panel. Briefly checking the pump over I found the fuse (15A) had blown and there seems to be <1 Ohm resistance across the pump itself (too low?).

To me this appeals like problem solved but would appreciate any insight as to what would have caused this (pump shorting out?) and some confidence that there's no need to go looking for another fault.

Thanks.

 

[Trundlebug]

Usually when you switch a circuit on a random lights illuminate elsewhere it's a sign of a poor earth somewhere, as the trickery tries to find an alternative path home to earth.

Although as you say the resistance of less than an ohm across the motor seems very low, I would nonetheless be tempted to check all the earth lines and junctions from both circuits all the way back to the battery.

You can check by resistance (from earth terminal of VHF and bilge pump separately back to battery) and also by visual and physical (shake / move / wiggle) means, as sometimes things alter with movement.

As removing the bilge pump fom the equation seems to have eliminated the problem I agree that's most likely the cause, however may be best to check earth resistance just to confirm - peace of mind.

 

[nigelmercier]

Sounds like a bad connection in the negative return wire. Your pump is probably OK, test it direct to the battery, perhaps via a 15A fuse.

 

[billmacfarlane]

I'm inclined to agree Trundlebug in thinking your problem is to do with bad earth connections. I had a similar problem last year when my VHF wouldn't switch on one day but would the next. Except the next day the chart plotter died followed by some of the internal lighting. The probem was(eventually) traced to the common earth buss bar which was located behind the fuse panel but under a leaky window which was dripping on to the bar. I'd start by checking the earth connections especially at common areas.

 

[SimonP85]

Thanks for the replies. Will certainly check the earth connections next time I'm down.

 

[superheat6k]

On a 12vdc system there is no earth, but commonly and for electrical safety / galvanic protection reasons possibly too diverse for this thread, the -VE is somewhere connected to the bonding circuit that is sometimes referred to as the 'earth' circuit. It is just as important that the current can flow cleanly back to the power source from each device as it can from it along the +VE, although strictly speaking the flow is from negative to positive.

Often several devices may share a common -VE return, and if the cable has a dirty connection somewhere or several devices are trying to return too much cumulative current along too small or long a cable, then the current flow reduces as the line resistance increases, this is when the problem manifests. For that reason it is better to run fully rated -VE returns from each individual device to a common busbar in turn connected by a far heavier cable to the negative -VE of the battery / power source. Daisy chained negatives are Ok for mast mounted lights where one or the other may operate, e.g. Nav Tri light OR Anchor light, but should routinely be avoided.