Stray voltage

[richard5933]

I've got a 12v water pump in the wet bay. It is fed by three different 12v+ feeds, one from each of the three switches upstairs (bathroom, kitchen, & cockpit).

There is an LED indicator light on the dash which lights when there is power to the water pump. Helpful because we like to turn off the pump any time we leave the bus, and this indicator is on our visual checklist as we go out the door.

Recently we've noticed a very faint glow of the indicator at all times. It might have been there for a while, but we just noticed it. So today I did some testing. I separated all the feeds, and found a few mV on each of them. Since they all go through the main wiring loom where both 120v and 12v live lines feed, I wasn't surprised to see this.

Here's where it gets weird...When I measure with the three feeds together, with all three switches in the 'off' position, I get 0.9v from them. I then pulled the fuse from the two rear switches and disconnected the wire going to the front switch at the switch. Still got 0.9v at the point all three join together to feed the water pump.

Is it possible to have this much stray voltage transmitted just by having the wires run through the loom?

My concern with all this is the result of having that phantom voltage going to the water pump all the time. My immediate solution will be to install a relay in the circuit. The coil in the relay will be triggered by the current water pump switches, and I'll run a new 12v+ feed directly from the fuse panel to the switch side of the relay. This should isolate the pump from any of the stray voltage floating around in the switches.

Still would like to learn more about this and figure out where the stray voltage is coming from.

Any thoughts?

[bevans6]

This is why it's against code (for buildings and probably for RV's) to have low voltage and 120 VAC in the same chase, duct, box, what have you.  It's very easy to inductively couple low voltages into ungrounded wires.  That low inductively coupled voltage will have almost zero current associated with it, so almost any load would reduce it to close to zero.  It won't do anything to the motor.  Adding the relay might even reduce it, as the voltage might just tie to ground through the relay coil.  I would stop running low voltage DC wires in the same chase as 120 VAC wires, it's bad practice.

Brian

[sledhead]

in the featherlite they kinda did the same thing . all wires ( 12v dc , 120v ac ) are in the same chaise but each wires are in a split wire loom so they stay separated kinda

dave 

[richard5933]

This is why it's against code (for buildings and probably for RV's) to have low voltage and 120 VAC in the same chase, duct, box, what have you.  It's very easy to inductively couple low voltages into ungrounded wires.  That low inductively coupled voltage will have almost zero current associated with it, so almost any load would reduce it to close to zero.  It won't do anything to the motor.  Adding the relay might even reduce it, as the voltage might just tie to ground through the relay coil.  I would stop running low voltage DC wires in the same chase as 120 VAC wires, it's bad practice.

Brian

Good information - thanks.

I'm going to claim 'not my fault' on this one - Custom Coach did the wiring back in 1974, and I guess this is just one more example of where they got it wrong. I'm slowly correcting all their wiring mishaps, but some are more difficult than others.

Aside from the shared wiring chases, there are multiple terminal blocks which have both 120v and 12vdc on neighboring terminals. I've tried to mark these best I can as I encounter them to avoid getting zapped, and eventually I hope to split them properly.

At least with the relay in place the water pump will be totally isolated from the problem.