44 DC Fuse & Circuit Breaker Types & Installation

44 DC Fuse & Circuit Breaker Types & Installation

Basically there are two types of direct current fuses and

circuit breakers.

The first provides Overcurrent Protection.

This type of protection is placed near the load (e.g. inverter, light, fan, pump)

and will break the circuit when the load draws more amperage

than the load is designed for.

An overcurrent protection device is meant to protect the load,

not the system or batteries.

Often the overcurrent fuse or circuit breaker will have a built-in

surge rating which allows it to handle more than the name plate

rating while a motor or other load starts up.

The delay can be up to ten seconds in some cases.

As an example, a 100 amp time delay fuse (Class R) may be able

to handle two or three hundred amps for up to ten seconds

before it blows.

Some circuit breakers have a 25% over name plate rating

for up to two seconds (this is called the time-delay curve in

some circuit breaker specification sheets).

In either case, a dead short-circuit will blow the fuse or trip

the circuit breaker, immediately.

The second type provides Short Circuit Protection (catastrophic).

This type of protection is placed near the battery(s).

It is designed to protect the battery(s) from a short circuit

in the system which could cause the batteries to over-heat or

in some cases, start a fire or explode.

In the grand scheme of things, batteries do not store much energy.

But in a dead short they can liberate a tremendous amount

of enegy in a few seconds.

Ratings:

The ability of a fuse or circuit breaker to clear a short-circuit

is rated as the Arc Interrupt Rating and given in amps.

This is where a protection device is needed that will break, also known

as clear or extinguish, a high amperage arc.

Some heavy d.c. rated circuit breakers are rated to interrupt an arc

up to 10,000 amps.

Class T fuses are usually rated at 20,000 amps in d.c. applications.

A good guide is to use the highest A.I.R. fuse or circuit breaker in

your system.

Always use a direct current rated fuse or circuit breaker

in a direct current system.

It is more difficult to clear a short-ciruit in a d.c. system than in

an a.c. system.

In an a.c. system the current is changing directions sixty times a

second, 120 times a second there is no current.

In a d.c. system the current is going in the same direction all of

the time.

Always use a fuse or circuit breaker that has the same or

higher voltage rating as your system.

The higher the voltage, the more difficult it is to clear a short circuit

arc in the device.

A Class T fuse will usually have a d.c. voltage rating of 125 v.d.c.

or 160 v.d.c.

It is not recommended to use a d.c. circuit breaker that has a very

high voltage in a low voltage d.c. system.

A battery system of 12 to 48 volts works fine with a circuit breaker

rated up to 160 volt d.c.

Occasionally you will see large molded case d.c. circuit breakers

with a d.c. voltage of 500 volts or more offered for sale.

These are usually magnetic circuit breakers and require a high-voltage

direct current to create a powerful magnetic field to trip the circuit breaker.

A problem and danger with using one of these circuit breakers on a

low voltage d.c. system is that in a short circuit situation it can take

so long to build up the magnetic field that damage can be done to the

system before the circuit breaker can clear the short, if ever.

When wiring a circuit breaker, attach the input (from the battery

bank) to the input connection marked on the circuit breaker.

A d.c. rated circuit breaker is designed internally to interrupt an

arc by pulling the contacts apart, and in some cases by

additionally physically blocking the arc.

There is a reason they have an input or line connection marked.

To be continued.

Prices and specifications subject to change without notice.

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