How to test an isolator

How to test a CAPAC Isolator.

From The Forum Host JOE3656


1) Do not disconnect the isolator. Do not disconnect the boat ground as a fix. Shore ground usually has a low voltage DC charge current. You need an isolator. Do not assume you don't need an galvanic isolator if you have an isolation transformer. 0.2 volts is enough to weaken the protection on the aluminum afforded by the anodes.


I had received help from Paul Fleury in MD (Marine Designs) on diagnosing an intermittent short in the ground from the shore connector to hull. 

Does the galvanic reading change on the capac meter when you are connected to shore power? Yes. The meter and the isolator are 2 different devices, but the meter warns the owner of insufficient protection.

An isolator is a filter for low DC voltages coming in on the ground. There are no known failures of the overbuilt CAPAC isolator itself.
A number of people have short circuited the ground wiring or disconnected these devices. They end up zincing the dock.

All boats that have AC wiring should have an isolator. I do not have good faith in the Guest brand, check out the quicksilver series from Mercury if you do not have one. The CAPAC system was the best one, but is no longer being built.

Inside a CAPAC is a big Sprague capacitor, a bunch of diodes (in a bridge circuit), a resistor and buzzer. The buzzer tells you if AC is in the ground circuit. You check an isolator with a ohmmeter (the ohms setting on the voltmeter). An increasing ohmage will occur in this circuit as the capacitor is charged from a power source. On the inside may be a wiring diagram.

One wire goes from the input ground (the dog leg) into the filter and then to the hull and ground. You must not have any connection from the shore connector on the boat to the hull before it goes into the isolator.

A dog leg is the sideways L shaped terminal on the shore power connector.

A correctly connected isolator, can be checked as follows.
1) Disconnect shore power. Look twice.
2) Connect one lead of the ohmmeter to the dog leg on the boat's shore power connector and one to the hull.
3) You should see increasing ohmage.
4) Short the dog leg connector to the hull with a wire
5) Reverse the leads and note increasing ohmage.

If you get zero ohmage, short the dog leg to the hull and try again, If it still fails, disconnect the isolator and test that.

A disconnected isolator (disconnected on both sides) will show increasing ohmage as the capacitor gains a charge from the ohmmeter. Short the two leads together and reverse them, the ohmage will again increase. (When the leads are reversed if the capacitor has a charge from the prior test, it may show zero ohms until the capacitor charge reverses, so short the leads together between each test.) When you reconnect the CAPAC isolator, it does not matter which lead is connected to the shore ground (it is a bridge circuit).

With the isolator disconnected, you should have no connection between the shore ground and the hull. If you do, fix the shorted connection from the connector to the isolator.

If this is fine, isolator is working correctly. You now need to check in interior wiring, with a neutral shorting to ground.

I have 18 feet of anode on my boat to get a 28ft express into the proper range in odd brackish fresh water. I will drop some of the anode next year. (2 6 ft pieces on the keel, and 1 6 foot piece split up on the rudders and trim tabs. 12 feet was not enough anode here, da*ned if I know why. The reading tells me what to do.  I also have a foot of anode in the bilge. 

Best of Luck Joseph