Back To Basics

What is a Safety Switch?

A safety switch is a device used to open and close a circuit . This may be done manually with an operating mechanism, or automatically with fuses.

A safety switch will always be found in its own enclosure. The enclosure provides protection to personnel against incidental contact with the electrical equipment. It also provides protection to the enclosed equipment against the environment.

There are two types of safety switches:

Non-Fusible

A switch with no associated fuses is referred to as a Non-Fusible safety switch, which has no circuit protection capability. It provides a means to manually connect and disconnect the load from its source. External overcurrent devices (such as Circuit Breakers or Fuses) provide circuit protection.

Figure 2. Non-Fusible Safety Switch and Separate Fuse

Fusible

A safety switch can be combined with fuses in a single enclosure. This is called a Fusible safety switch. The switch provides a means to manually open and close a circuit and the fuse provides overcurrent protection.

Figure 3. Fusible Safety Switch

Purpose of a Safety Switch

Safety switches are generally used for two main purposes:

Let’s take a moment to consider each purpose briefly.

Service Entrance

A service entrance is a point at which electricity first enters a building. There are times when a safety switch is used at the service entrance point to disconnect power to the whole building at one location. Consider an existing commercial building. If additional electrical service is required as a result of building expansion, an electrical contractor will usually install safety switches.

Figure 4. Adding Circuits to an Existing Commercial Building

Means of Disconnection

The National Electrical Code® (NEC) requires that a “disconnecting means shall be located in sight from the motor location and the driven machinery location” (Article 430-102b). The NEC defines “in sight” as visible and not more than 50 feet (15.24 meters) from the motor or machinery.

With the power removed, the operator can safely work on the machinery. There is no need to worry about touching live electrical components, or having the motor accidentally start.

Figure 5. Safety Switch Used as Power Disconnection Means

Circuit Protection

Why do we need circuit protection? Circuit protection prevents damage that would otherwise be caused by Overload (or Overcurrent) and Short Circuits. Overcurrent protection devices such as fuses and circuit breakers are used for this purpose.

An overcurrent occurs when too many devices are operated on a single circuit, or a piece of electrical equipment is made to work harder than designed. For example, a motor rated for 10 amps may draw 20, 30 or more amps in an overload condition.

A package has become jammed on this conveyor, causing the motor to work harder and draw more current.

Overloaded Motor

Damage will occur to the motor in a short time unless the problem is corrected or the overcurrent protection device shuts down the circuit. Of course, motors are not the only devices that require circuit protection for an overcurrent. Every circuit requires some form of protection against overcurrent.

Heat is a major cause of insulation failure on any electrical component. Insulated wire exposed to high levels of heat suffers insulation breakdown, and it may even flake off, exposing the conductors.

Figure 6. Insulation and Heat

When two bare conductors touch, a short circuit occurs and resistance drops to almost zero. Short circuit current can be thousands of times higher than normal operating current.

Figure 7. Short Circuit

Ohm's Law gives us the relationship between current, voltage, and resistance. Consider a 240-volt motor with 24 ohms of resistance. It would normally draw 10 amps of current.

I = E / R

I = 240 volts / 24 ohms

I = 10 amps

If a short circuit occurs, the resistance drops. Suppose the resistance drops to 24 milliohms, which is a reasonable figure. This means that the current will rise to 10,000 amps.

I = E / R

I = 240 volts / 0.024 ohms

I = 10,000 amps

The heat generated by this high current will cause extensive damage to connected equipment and conductors. For this reason, when a short circuit occurs, this dangerous current must be interrupted immediately.