RP 156A - Electrical Circuit Protection Components: Introduction

The primary role of a circuit protection device is to protect a wiring system from over-current induced thermal damage by opening the circuit. Thermal damage may include degradation of insulation or conducting material to the point that a short circuit or open circuit occurs. These devices include; mini fuse and breaker, ATC fuse and breaker, Maxi fuse and breaker, glass fuse, PPTC and fusible link wire (see Glossary of Terms  ). All rely on heat induced by the over current for correct operation.

The main difference between a fuse and circuit breaker is that a fuse is a one time only device and a circuit breaker can open a circuit multiple times. The fuse relies on an internal element melting in an over current event and cannot be used again. A thermal circuit breaker uses an internal bimetallic strip that reacts to heating during an over current event, opening a set of contacts and interrupting the circuit. When the over-current condition is corrected, the bimetallic strip will cool allowing the contacts to close and restore circuit continuity. A magnetic/hydraulic circuit breaker consists of an inductive coil that produces an electro magnet when current passes through it. The magnet attracts an armature which opens or closes the circuit. The coil/armature mechanism will ultimately cost more than a thermal breaker's bimetallic strip.

The devices described are intended to protect the wiring system from damage due to a short circuit or overload induced condition. A short circuit takes place when the circuit has continuity to ground before the circuit load or an internal short occurs in the load. This permits a large current to flow in the circuit, causing potential damage to the wiring system or other elements in the circuit. An overload takes place when the current in a complete circuit exceeds the design limitations of the circuit protection device. A properly applied circuit protection device will open the circuit before damage to the wiring system can occur.

All protection devices except fusible links have voltage ratings and current ratings. The circuit protection device is designed to support the rated current rating under steady state operating conditions and at room ambient temperatures (25°C). The voltage rating of the device represents the maximum system voltage in which the over current device can be installed. The system voltage should not exceed this value for proper operation of the device during an over current event. High and low ambient temperatures affect the performance of the thermal protection devices. Temperatures are a design criteria used by the OE when selecting the proper circuit protection device. Manufacturers of the circuit protectors will determine the devices' current rating using a controlled temperature of 25°C. TMC recommends that the thermal circuit protector be operated at no more than 75-80 percent of its marked current rating. The magnetic/hydraulic circuit breaker rating should be determined by using following general guideline:

1. Determine what the normal (stabilized) operating current of the breaker will be.
2. Use the list below, to determine the "safety factor" precluding surge-induced tripping.

   Typical safety factors:

   • 1.20 on resistance loads
   • 1.75 on inductive, transformer loads
   • 1.5 on linear power supply circuits
   • 2.0 on switching power supply circuits
   • 1.25 on motors

   Final current rating = (normal operating current) x (safety factor).