Polarity & Open Ground

BlueJ · #1 (**permalink**) 10-30-2008

My boats AC panel has the traditional push to test button and light for reversed polarity test.
The light does not work - I have replaced the LED and tested the switch.

When I test the resepticles on the boat with a polarity tester it shows the polarity is OK but that I have an open ground (whetever that means).

Could the open ground be stopping the polarity tester from working and how do I find where the open ground is coming from?

I had the marina check the supply and my shore power cable and both are OK.
Thanks - BlueJ

sailingdog · #2 (**permalink**) 10-30-2008

You need to check the AC wiring on your boat. Most AC panels I've seen don't have a test button, instead having two LEDs, one of which indicates the panel is live and the other indicates reverse polarity.

One thing that you should check is the shore power outlet directly. Get a pigtail that converts the 30 Amp shore power male fitting to a female 110 AC house hold fitting. Put your polarity tester in there and see if the problem exists at the post... if it is there... then the problem many not even be on your boat.

US27inKS · #3 (**permalink**) 10-30-2008

My boat has a polarity test button like yours. All it does is test the voltage between the hot and the ground. If it's hot, you're ok. If not, you have neutral and hot swapped. With no ground, you get no test.

Fix your ground problem and you should be ok.

sailingdog · #4 (**permalink**) 10-30-2008

US-

If the ground connection problem is at the post, nothing he does on his boat is gonna fix it... gotta know if the problem is on the post or the boat.

FarCry · #5 (**permalink**) 10-30-2008

Welcome to Sailnet BlueJ.

Have you confirmed that your polarity tester is functioning correctly, by testing it on a known source? I would suggest doing that first if you have not already. I have seen stranger things...

Did you check every receptacle on your boat? There may be a problem at one of the first connections causing the open ground. Before going back to the pedestal you might confirm that the recptacle closest to the entry of the A/C power on your boat does indeed indicate an open ground. If it does then you should check the dock recptacle with an adapter as Sailingdog reccommends. The adapter is not necessary if you are comfortable working with electricity, you could use a conventional meter or even a 120 volt test light. On the dock pesdestal receptacle there will be three arched "slots" one of which has a slight 90 degree dog leg in it, which is the ground. If you insert a test probe into that slot and go one slot counterclockwise (hot) you should have voltage. If you do not then the problem is at the dock. If you do have an indication of voltage then I would suggest repeating the same procedure with your shore power cord plugged in and confirm that a proper ground is getting to your boat. If it is getting to your boat recptacle but no further I would check the wiring connection on the recptacle itself by removing it and looking at the wires and tugging on them to confirm they are secured, have the pwer off!!!!!!!

IF YOU ARE NOT COMFORTABLE DOING ANYTHING SUGGESTED ABOVE---DON'T DO IT

Electricity, especially around water can be lethal. Use caution, or get a skilled tradesman to do it.

BlueJ · #6 (**permalink**) 10-30-2008

Thanks for the responses.
The AC panel does have a push to test (not 2 LED's).
The marina checked the power supply when I was with them and they showed me that the power at the post and my shore cable were OK.
I also checked every power output on the boat and they all showed the same - polarity OK - open ground.
Can anyone help me with ideas of how I check on the boat for the open ground?

FarCry · #7 (**permalink**) 10-30-2008

Is it safe to assume that you are in the US and have a single 30 amp shore power cord?

My suggestion would be to start at the A/C panel with a A/C tester. Ideally you will have a three wire cable (white, black and green) coming from your boat receptacle to the panel. If you can locate the cable attachment point at the panel you then need to put your tester probes between the black and green (hot and ground). If you see nothing then you need to go back to the actual shorepower receptacle on your boat and do the same thing. If you are confident your shorepower cord is correct then it must be the actual receptacle, or the wiring connection to it that your cord is connected to.

If you see 120 volts then there is a problem between the panel and the receptacles. Follow the wiring from the panel to the first receptacle. Remove the receptacle so that you can access the wiring on it. Using your tester place one probe on the ground wire (green or bare) and one on the black. If you see voltage then follow the wiring downstream to the next one. If you find a group of wires and a wirenut or a crimped connection, wiggle it around and see if that is where you are having connection problems. If that is the problem, make a better connection.

There are so many variables that could be causing this that I could type pages of things for you to try. I suggested what I think is the most likely. There is always a chance the outlets where wired with a two wire cable instead of three. I have seen people not bother to hook up ground wires before. I have seen rodents chew thru wiring too. Have you looked at the physical condition of the wiring from the boat recptacle to the A/C panel?

What kind of boat is it? How long have you owned it? What prompted you to test it for proper wiring, were you having some sort of problem?

I hope you figure out the problem and fix it properly. The ground system is sort of the "liferaft" of wiring. If all else fails it is there to save you and trip a breaker.

US27inKS · #8 (**permalink**) 10-30-2008

It's time for a multimeter. You need to test the resistance (ohms) between the shore power receptacle on the boat and the AC panel. There should be three wires. Hot, neutral, and ground. Hot and neutral are the ones that do all the work. The ground is there for safety in case there is a short in whatever you have plugged in.

Here is a picture of common plug configurations. Boatowner's Illustrated Electrical ... - Google Book Search

You should have 120 volt 30A 3 wire. If you have something different, it should be in figure 9.2. The pictures are marked with the white (neutral) and green (ground) wires. The other wire is black (hot). Don't worry if the wire colors on your boat don't match. Mine has blue, red and black. Just go by the pin positions on your plug. Match the pin for the ground with the wire color on your boat and test the resistance to your AC panel. You will likely find the problem in the AC panel, or at the shore power receptacle.

If you get lost, let me know and I'll try to walk you through it.

Edit: almost forgot. when testing resistance, the shore power needs to be unplugged. don't try this with the ac power on.

sailingdog · #9 (**permalink**) 10-31-2008

Before checking between the shore power receptacle and the AC panel, I would recommend you check the three wires on the shore power cable itself to make sure that the cable is good. Check each of the pins on the male plug against the three female sockets on the other end. The meter should read open for two of the three for each pin and near zero for the third.

If the cable is good, then you should plug the cable back into the boat and bring the male end into the cabin and use it to check the shore power inlet to the main AC panel.

If the connections from the male end of the shore power cable to the main AC panel are good... check to the first outlet on the AC side of the boat. If all of these connections check out...then the problem is at the Shore power post. If not, then there is a problem between the AC panel and the first outlet on the boat.

BlueJ · #10 (**permalink**) 10-31-2008

Thanks again for the responses. I will try to trace the wiring from the recepctacles back to the AC panel and check they are 3 wire. In response to some of the questions asked:
- The boat is in the USA and has 30 amp 120 V shore power.
- I bought the boat about 2 months ago - its a Aloha 32 sailboat (1984)
- I started checking the wiring because the polarity tester on the AC panel was not working. I replaced the LED and checked the switch, all OK.
- I then thought I would check the polarity of the receptacles with the open ground result.

One more question - would the open ground stop the polarity test lamp working?
Thanks to all again!