Part 3: Multimeter Testing Of Your Boat For Corrosion Potential

Previously, we looked at the two principal forms of destructive corrosion aboard boats: galvanic corrosion and stray current corrosion. With this background, you may now wonder how exactly you check your own boat if you suspect visible damage is attributable to either of these corrosion modes.

Testing is something you should not put off. Additionally, electrical stability testing should be an ongoing process supported by detailed record-keeping, beginning with baseline testing of various parts of your boat and the surrounding seawater, carried out well away from other boats in an isolated cove.

Necessary equipment

1. A digital multimeter. This high-impedance instrument should have a DC millivolt range. The better digital meters have a self-ranging feature, so there will be no need to set it.
2. A ground electrode. Heavy-gauge copper wire—10- or 12-gauge—with an alligator clip at one end and a jack plug on the other end to fit the black socket on your multimeter.
3. A record book. Also, a helper makes the testing process much easier.
4. Recommended: a schematic drawing of your boat’s electrical system, showing all the branch circuits, the location of all breakers and/or fuses and the bonding system with all fittings connected to it. You might have to draw this yourself. A good starting point is the generalized drawing in this article.

Multimeter Connections For Testing Stray Current At The Slip

You can use the multimeter alone to test for galvanic or stray current corrosion in your slip with standard probes. Connect the multimeter using the following procedures depending on your situation, then run the test procedure:

1. For boats without galvanic isolators or isolation transformers, temporarily disconnect the shore AC green wire from the required connection to the boat’s DC ground.
2. For boats with galvanic isolators, temporarily disconnect the shore AC green wire from the connection to the galvanic isolator.
3. For boats with isolation transformers, disconnect the boat’s green wire from the neutral wire at the output of the isolation transformer.

Multimeter Testing Procedure

1. Set the current selector at the highest range.
2. Connect the multimeter (set on the highest current range) in series with the AC green wire and the DC connection.
3. Reduce the current range until a meaningful current reading appears in the display (at least two non-zero digits).
4. Check for both DC and AC currents.

Interpreting The Results

If testing shows that AC current is over 1 milliampere or DC current is over 0.01 milliampere, further investigation is required. To locate the source of stray-current corrosion, tum off all AC and DC circuits and selectively tum them on one at a time to locate the source of the stray current. Then troubleshoot that circuit.

Multimeter Testing For Stray Current Within The Boat

1. Turn off all DC breakers (main and branch).
2. Remove the negative DC cable from the battery terminal.
3. Connect the DC ammeter in series with the house battery’s DC negative cable and the negative terminal on the battery.
4. With all DC branch circuit breakers and switches off, tum on the DC main breaker. You should not measure any DC current.

Multimeter Testing For The Bilge Pump

1. Connect your multimeter as in Steps 1-3 above.
2. Hold down the bilge pump float (so the bilge pump level switch if open).
3. Turn on the bilge pump circuit breaker. If you measure any DC current, you have either defective bilge pump wiring or a defective float switch that is letting DC current flow into the bilge and back to the battery.

Stray current, if present, will eventually corrode the bilge pump wiring in half and prevent the bilge pump from operating.

Multimeter Testing For Stray Current From Branch Circuits

Other circuits can be similarly checked. Make sure the DC device is turned off, and then tum on the branch circuit breaker for that device. With the device off, if you measure any DC current you have defective wiring.

Stray Current Testing On Boats With Bonding Systems

Note that it is difficult to test for stray DC current if the boat has underwater sacrificial anodes connected to the bonding system, since they will induce a current as they are being destroyed. Before testing or measuring stray current, disconnect any sacrificial anodes connected to the bonding system.

Next time: Testing both AC and DC circuits at the slip using a multimeter and a reference electrode.

Source:

Marine Electrical Systems, AC/DC-Electrical Circuits, Grounding, Lighting, & Safety; United States Power Squadrons; 2008.

Learning More

Does marine corrosion seem complicated to you? Are you not sure where to start in applying best practices to your boat? United States Power Squadrons has a solution for you. Marine Electrical Systems is an elective course dedicated to onboard electrical power–-both AC and DC. You will learn about electrical circuits, wiring, terminating, and routing wiring for safety and trouble-free operation. You will learn how to use multimeters and tools to check, diagnose and repair electrical problems. You also will learn about galvanic corrosion, stray currents, isolation, grounding, and safety of electrical systems.

The course material is organized in seven chapters and is intended to be presented in the classroom over nine two-hour sessions:

Day 1 – Introduction, Chapter 1: Properties of Electricity

Day 2 – Chapter 2: Boat Electrical Wiring Practices

Day 3 – Chapter 3: Direct Current Power

Day 4 – Chapter 4: Alternating Current Power

Day 5 – Chapter 5: Galvanic and Stray Current Corrosion

Day 6 – Chapter 6: Lightning Protection

Day 7 – Chapter 7: Troubleshooting Part 1; on-the-boat optional

Day 8 – Chapter 7: Troubleshooting Part 2

Day 9 – Review and Examination

This course is the best way to learn about electrical factors that can save you from sustaining considerable damage, especially if you berth your boat at a marina, or if you make any repairs or modifications to your OEM electrical system.

If you are interested in taking this class, indicate your interest using the contact page.