Home . Articles . How-to . Boat Doctor . Why did the power go out?

Why did the power go out?

2011 May 5

Dear Boat Doctor,
I recently had a very strange electrical problem on my C&C 44 that I don't quite understand. I was on a passage, things were going great, and suddenly I lost all electrical power. Everything just went dark. One of my crew is an electrician and luckily he discovered the problem was that the ammeter blew out. He bypassed it and all was well. Does this make sense? It seems strange to me that the voltage supply to my panel would pass through the ammeter and that power would be disabled if the ammeter had a problem.
Fred Allen
Stamford, Connecticut

Dear Fred,
Your problem and diagnosis does make sense, and I can tell you a little about what may have happened. I'll start with a little electrical review. An ammeter measures current (in amps); current is a measure of the actual flow of charge (in the form of electrons) in a circuit, analogous to the flow rate in plumbing. To measure current in a circuit the ammeter must be placed in series (in-line) with the load, this fact is why you saw power first go through the ammeter before going to your panel.

For some reason your ammeter failed, causing the circuit to open, which stopped the voltage going to your panel. In reality, an ammeter consists of a meter movement (a resistor of sorts) connected in parallel with a scaling resistor called a shunt resistor. The current flow in the circuit is divided between the two parallel legs of the meter. Failure of the meter movement alone would not cause the problem you describe; the shunt resistor would need to open, or more likely some interconnect within the meter failed.

An ammeter of the design you had is called an internal shunt. The shunt resistor is inside the meter housing, and the meter itself needs to be connected in the circuit being measured. I prefer an ammeter design with an external shunt, but my preference is purely based on mechanics and the ability to examine and troubleshoot any problems. With an external shunt, power is connected through the shunt resistor and a small signal cable is connected to the ammeter to indicate the current flow. The shunt resistor is a robust chunk of brass and copper with large terminals and any mechanical or electrical problem with it would be obvious. The ammeter can actually be removed and current would still flow uninterrupted.

The only negative to an external shunt ammeter is a bit more cost, and it takes a bit more space to install the shunt. I think these items are a worthy trade-off for a more robust shunt and the ability to examine the circuit easily.