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Old 29-04-2014, 17:06   #1
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Shocked from Touching A/C Distribution Panel

Can someone help me figure out why I get shocked when I touch the A/C distribution panel screws?
I put a volt-meter up to the screws and a ground, and it read out 12 volts. I don't understand how 12 volts can give such a jolting shock! I don't feel shocked when I touch 2 battery terminals.
This boat isn't plugged in to A/C power, and my inverter is not plugged in. This panel says "Blue Sea Systems" "120v 60hz distribution"
"a/c main" the breaker is set to off.
"reverse polarity"
and "mermaid" (the heat pump).
All the breakers on this panel and the other A/C panel are switched off, yet, whenever I touch the screws, I get a jolt.

Is this probably constantly leaking electricity from my batteries into the wooden bulkhead that this panel is screwed into? Should I remove some of the powered wires from the back of it?

The PPO was an engineer and he installed the heat pump, and it looks like he did a professional job, but, it shocks me when I touch the screws, or wipe it down w/ a rag while spring cleaning.
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Old 29-04-2014, 17:30   #2
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Re: Shocked from touching A/C distribution panel

Check your inverter's AC output terminals. I'll bet you will find it is on even though you think it is off. 12V won't shock you. Disconnect the heavy red wire from the inverter to the battery to make sure it is shut down and then see if you are still shocked.

In addition you have some AC fault to ground that is not being grounded properly That is causing the shock. I suspect your inverter's ground connection is loose.

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Old 29-04-2014, 17:45   #3
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Re: Shocked from touching A/C distribution panel

Could there be a small inverter which you haven't yet found? Did you check for AC volts as well as DC with your meter?

I think it's unlikely but maybe just possible for a fault in an adjacent vessel's system to leak 120V to yours through the water - but I'm probably wrong.
Presumably disconnecting your batteries would make no difference if this were the case.
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Old 29-04-2014, 17:48   #4
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Re: Shocked from touching A/C distribution panel

If disconneting the inverter does not solve it which was my first guess too. Check voltage between panel screw and DC ground as well as voltage between panel and AC hot and neutral. (AC not connected of course).

It might be a ground loop from another boat or dock wiring), coming in on the DC ground, which many times the AC ground is connected too (per ABYC). AC ground would be grounded to the panel frame too.

Verify that power, and any communications cables are disconnected.

Do you have a shore/generator switch. If that can be turned off switch it to off.

If still a voltage present, then turn all 12V off and turn battery switch off and check again.. Could also be a loose wire touching the AC panel as its aluminum I think.

Still got voltage then you have a ground loop from outside the boat. I would talk to the Harbour master after that. It's alas not at all that uncommon.
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Old 29-04-2014, 18:03   #5
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Re: Shocked from touching A/C distribution panel

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Originally Posted by sailorchic34 View Post
If disconneting the inverter does not solve it which was my first guess too. Check voltage between panel screw and DC ground as well as voltage between panel and AC hot and neutral. (AC not connected of course).

It might be a ground loop from another boat or dock wiring), coming in on the DC ground, which many times the AC ground is connected too (per ABYC). AC ground would be grounded to the panel frame too.

Verify that power, and any communications cables are disconnected.

Do you have a shore/generator switch. If that can be turned off switch it to off.

If still a voltage present, then turn all 12V off and turn battery switch off and check again.. Could also be a loose wire touching the AC panel as its aluminum I think.

Still got voltage then you have a ground loop from outside the boat. I would talk to the Harbour master after that. It's alas not at all that uncommon.
+ 1

Tighten up EVERY mains connection and battery connection especially the ground. A loose ground does strange things.
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Old 30-04-2014, 00:46   #6
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Re: Shocked from touching A/C distribution panel

the panel itself should be grounded to the boat. which would stop shocks. and blow fuses / breakers instead.

so once you find the problem of why it's getting hot. ground the panel as well.

are you measuring 12v ac or dc? what is your meter set to?
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Old 30-04-2014, 08:27   #7
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Re: Shocked from touching A/C distribution panel

Any of the 4 screws on the A/C distribution panel measure out 12 volts DC when connected to the digital multimeter and the other probe is on any ground, or any of the grounded screws on the other panels, including the a/c outlet nearby.
All inverters are unplugged.
The digital multimeter detects 0 volts A/C when set to read A/C voltage, no matter what the combination. I plugged in my A/C inverter and tested that it works, & then unplugged it for the other tests. It works.
This could be the short that's slowly draining my batteries, couldn't it?
The ground wire is well connected. I checked it earlier, but will recheck it. It's green .
I'll probably end up trying to remove the positive and just putting a cap over it w/ tape and mount it so it stays dry, or something.

I'm not an elec engineer, but something here seems real strange. If I touch the screws on this one panel, I get shocked. The screws seem to be connected to the positive terminal of the battery. I just checked. Red probe touching screws, black probe touching ground in a/c outlet, and it read out 12.34 volts, so, the screws are positive. Normally, screws are negative, or grounded and the ground connects to the negative terminal of the battery.

But this Mermaid air conditioner's panel has it backwards. It also happens to have a breaker-switch labeled "reverse polarity" that is bridged to the "a/c main 30 amp" breaker, so they both go on or off at the same time.

This panel is mounted right net to another panel that has "a/c main, battery charger, refrigeration" and it also has a bridged reverse polarity on the a/c main breaker switch. But this other panel doesn't have hot screws, so to speak. I'll be the current is leaking accross the wood from the pos screws on one panel to the neg screws on the neighboring panel and draining my batteries. Since I don't use a/c (any time soon) or need it, I may as well disconnect something & see what happens.
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Old 30-04-2014, 08:39   #8
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Re: Shocked from touching A/C distribution panel

You're measuring 12V DC... have you taken an AC measurement?
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Old 30-04-2014, 09:16   #9
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Re: Shocked from Touching A/C Distribution Panel

I think your not in a marina and not on shore power, your anchored out, am I correct?
It makes a difference.
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Old 30-04-2014, 09:53   #10
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Re: Shocked from Touching A/C Distribution Panel

If you are hot and sweaty enough you can be shocked by 12v dc. To me, it feels like someone it sticking you with a little pin.
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Old 30-04-2014, 10:18   #11
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Re: Shocked from Touching A/C Distribution Panel

stray a/c isn't the problem. it's been measured.
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Old 30-04-2014, 10:20   #12
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Re: Shocked from Touching A/C Distribution Panel

are you sure this isn't just static electricity?
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Old 30-04-2014, 11:39   #13
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Re: Shocked from Touching A/C Distribution Panel

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Originally Posted by a64pilot View Post
I think your not in a marina and not on shore power, your anchored out, am I correct?
It makes a difference.
Yeah, like five States wondering why the lights went out

Check for cables before you drop the hook next time, innit?

Eleventeen kV up the chain... tickles
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Old 30-04-2014, 11:45   #14
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Re: Shocked from Touching A/C Distribution Panel

This is why you are not supposed to install good wool carpets on a boat. (G)

Really, you need a LOT of 12v power to feel anything from it. If you just get one jolt when you touch the panel, and touching it again doesn't give you another jolt, then you are somehow just discharging static electricity.

If you get repeated jolts every time you touch it, there's a ground fault.

So which is it? Static discharge from those nasty wool carpets? Or ground fault that bites you every time you touch the screws, time after time?
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Old 30-04-2014, 12:05   #15
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Re: Shocked from Touching A/C Distribution Panel

I hope you are not repeatedly getting shocked to test each possible solution - a low voltage test light will be less painful.
Quite the mystery - a couple pictures might help.
Most panels are plastic of some type, is the frame that the screws are into metal?
Are the 12 volt ground, AC ground and AC neutral all on the same buss somewhere ( with all power off check continuity from 12 volt ground to AC neutral). With a bad connection or chaffed insulation you could be getting 12 volts into the AC side of the panel.
12 volts with enough amps can hurt you - and unknown issues behind the electric panel rarely are good.
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