Circuit Protection

Why is Circuit Protection so important on an Electric Boat?

I have been Santa Barbara’s Marine Electrician for 11 years and this is one of the first concepts I explain to a new customer.    Most people believe that circuit protection is installed to protect the electrical appliance.    This is true and the appropriate fuse should be picked to protect an appliance from abnormal operating conditions.    However the true reason for circuit protection is to protect the entire circuit and the circuit breaker or fuse is sized to protect the smallest wire size in the complete circuit.    ABYC code states that circuit protection must be put at any source of power.   Sources of power include batteries and anywhere a wire is dropped in size.   Self regulating charging sources are exempt from fuse protection at the charger side.

The only way I agree to take on the responsibility of being the marine electrician of a boat, is if they agree to installation of fuses as required by ABYC code.   This simple rule has allowed me to operate without incident for 11 years in the Santa Barbara Harbor.

Explain capacitance, resistance and inductance and how they relate to circuit protection?

All electrical circuits have three main properties: capacitance, resistance and inductance.

Capacitance is the property of a circuit that opposes a change in voltage. Capacitance does not pose any issue with circuit protection because with a battery system we are mainly focused on current protection.    Voltage is more or less clamped to battery voltage potentials.

Resistance is the property of a circuit that resists the flow of current through a circuit.  We are very concerned about resistance in a circuit.   High resistance causes a voltage drop, inefficiency and consequently heat.    Our circuits are designed to be low resistance with large conductors and currents well within the ampacity of those conductors.   Conductors include battery cables, battery lugs, crimp styles and anywhere current must pass.

Inductance is the property of a circuit that opposes a change in current. We are very concerned about inductance with DC circuits.   DC circuits have far more inductive properties than AC circuits because the polarity and voltage never change.

How do AC and DC circuits differ?

In an AC circuit, the polarity and voltage change 60 times per second in North America – 60 HZ and 50 HZ in other parts of the world.   Breaking an AC circuit is simple because 60 times a second both current and voltage go through zero and the circuit naturally will fall apart.

A DC circuit is one giant inductor!   Once current starts flowing in one direction it does not want to stop.    Think of it like the electrons have momentum.    This makes it a very powerful force which has to be protected differently.    When a DC circuit is broken under load, the circuit wants to continue and will in the form of an arc.    Switches are therefore used as isolation switches to isolate two or more battery packs.   The circuit protection must be done with a circuit breaker.    A circuit breaker can open the contacts faster than the arc can be created, thus eliminating any issues caused by inductance in the system.

Inductance rises proportionately to voltage and to current.     Higher Inductance circuits require more separation of the contacts of a circuit breaker and longer fuse filaments in order to insure an arc cannot be sustained.

What are the ratings for your circuit protection devices?

All Propulsion Marine Electric Drives come with circuit protection rated at 175 volts DC.     Many electric drives still employ circuit protection rated at 32, 42 or 48 volts.    A 48 volt system charges at 58 volts so these fuse devices are inadequate for electric propulsion.    The next voltage range is 175 volts which gives us a great safety margin!