Circuit Protection Primer

Why do I need circuit breakers?

Appliances, machines, and tools that use electricity can not distinguish between normal electrical loads and overloads. They are not built with circuit protection in mind, they simply use electricity to function. Most electric devices will destroy themselves, and possibly the utility company, your house or you if they are not connected to circuit protection devices such as fuses or circuit breakers.

How do they work?

To achieve optimum circuit protection the correct combination of circuit protection devices, and proper sizing and safe routing of wire must be set in place. Circuit protection systems limit overloads to a single circuit, and minimize the danger of fire in both the equipment, and the circuit leading to and from it. A protection device is generally used at any point in an electrical circuit where the size of the conductor is reduced, unless the circuit breaker upstream provides enough protection for the smaller wire as well.

Essentially the circuit protection device must sense and isolate the overload from the power source so that other circuits will continue to function properly. It then disconnects the circuit from the electrical distribution system before the wire’s insulation is melted away. Circuit breakers are available in different delay curves, which delays when the breaker actually trips or disconnects the circuit from the power source. This allows the overload of electricity in the circuit to dissipate before the breaker trips. Breaker trip delays must be long enough to avoid frequent annoying tripping, but short enough to disconnect the circuit from the power source before a dangerous situation is imminent.

Are Circuit Protection Devices Effected by Environmental Conditions?

When designing electrical circuits environmental factors definitely need to be kept in mind. All circuit protection devices are subject malfunction, to some extent, by adverse environmental conditions. High humidity or a corrosive atmosphere can cause circuit protectors to fail over time. The most harsh environment is one that is surrounded by sand and dust. Unsealed circuit protection devices easily collect sand and dust, eventually impeding their operation. These situations require the circuit protector to be sealed from the surrounding environment. It is also a good idea to establish procedures where the protection device is regularly tested to insure it is functioning properly. In addition to these precautions, explosive environments require a special system design to insure that any open flames or sparks will not occur. It is often necessary to use hard conduit, sealed circuit protection devices, and switch panel boxes.

Heat, high vibration, shock, and improper mounting can also damage and cause circuit protection equipment to trip without an electrical overload. A high quality magnetic breaker uses a balanced armature, which helps minimize the effects of shock and vibration.

What types of circuit protection devices are available?

There are four main types of circuit protection devices including:

1. Fuses are most commonly made of lead alloy metal links enclosed within a glass tube. They are dependent on the metal link melting to shut down the electrical circuit. Fuses are used frequently for circuit protection and work fairly well, yet their design has a few intrinsic problems. Their exact tripping or melting point is highly variable making them more reliable for low amperage functions. Also they must be replaced every time their is an electrical overload because of their self destructive nature. Fuses also have a tendency to wear out over time due to fuse element deterioration, which causes mysterious fuse failures. Fuse element deterioration occurs when the metal link receives chemical and physical stresses due to repeated short duration electrical overloads that are not serious enough to cause the link to actually melt.

2. Thermal mechanical circuit breakers are dependent on a rise in temperature of the thermal sensing element to cause the breaker to trip. When the pre-determined temperature of the breaker is reached a piece of metal is warped triggering a release mechanism that disconnects the circuit from the electrical source. Thermal breakers have been known to trip because of changes in the ambient air temperature, so manufacturers now produce a temperature compensated thermal breaker where a thermal responsive element is used. This element causes the temperature calibration of the breaker to fluctuate above or below the preset room temperature calibration, which compensates for changes in ambient air temperature. Due to their temperature sensitive nature most low cost thermal breakers are best suited for protecting wire in low voltage circuits. When selecting the correct thermal breaker one must take into account the wire rating, ambient operating temperature, allowable voltage drop, and the heat sink provided.

3. Magnetic hydraulic circuit breakers come in two types: sealed and nonsealed. Sealed magnetic breakers are less affected by harsh environmental conditions, whereas nonsealed breakers can handle higher electrical requirements yet are more effected by the surrounding environment. Magnetic breakers operate on the solenoid principle where a movable core held with a spring in a tube and damped with fluid is tripped by changes in the magnetic field of a series coil. The greatest feature of magnetic circuit breakers is they rely on electrical current alone, which is not really effected by changes in ambient air temperature, to cut the circuit off from the power source. These breakers are also versatile and can be used in coordination with other forms of circuit protection.

4. Ground fault protection detects the presence of current to ground, and interrupts the circuit before electric shock becomes fatal. This circuit protection device is not required for all home circuits in the U.S. because of the partial protection provided by the third ground wire system. Ground fault protection is extensively used in Europe because most electric installations are 230 Volts, and the third wire ground is not used.

The choice of which to use depends on the specific application that needs protection, its electrical specifications, space available, environmental constraints, and customer preference. The Underwriters Laboratory (UL) recognizes two classes of circuit protection including "Listed" branch circuit breakers and "Recognized" appliance circuit protectors. Listed branch breakers protect wiring and/or the electric equipment. Recognized appliance protectors specifically protect the equipment being used.

Certain electrical applications require specific UL listings so it always a good idea to check with a professional electrician or the National Electrical Code before installing circuit protection devices or other electrical equipment. Rainshadow, Inc. carries only UL listed magnetic-hydraulic circuit breakers because of the consistent protection they offer over a wide range of environmental factors.