Basics of Safety Switches


#1

Introduction

A safety switch is a common type of enclosed switch. Safety switches are generally used for two purposes:

  1. As a disconnecting means for a service entrance
  2. As a disconnecting means for motors
    1

In either case, a safety switch may incorporate provisions for a fuse for overcurrent protection. The safety switch enclosure provides a degree of protection to personnel against incidental contact with live electrical equipment. It also provides protection for the enclose equipment against specific environmental conditions
1

Parts of saftey switch

  1. Cover interlock
  2. Tangential knockouts through 600A for easy conduit lineup
  3. Quick-make, quick-break operating mechanism that ensures positive operation
  4. Provisions for T, R, J, H, and K class fuses (T & J 100-600A)
  5. Generous wiring gutters that meet or exceed NEC wire-bending space requirements
  6. Visible blade, double-break switch action
  7. Positive 2 or 3 point mounting
  8. Highly visible red handle grip
  9. Informative door labeling which includes replacement parts list
  10. Handle and cover padlocking provisions
  11. Side-hinged door that opens 180 degrees for easier wiring
  12. A unique enclosure design that adds rigidity and strength. Its rolled edge prevents cuts and
    scrapes to conductors and to installer’s hands

Switch Symbols

1el

Non-Fusible Safety Switch


A safety switch with no associated fuses is referred to as a non-fusible safety switch. A non-fusible safety switch has no circuit protection capability. It simply provides a convenient means to open and close a circuit. Opening the circuit disconnects the load from its source of electrical power, and
closing the circuit connects the load. Circuit protection must be provided by external overcurrent devices such as a circuit breaker or fuses. In the following illustration, power is supplied to a motor through a non-fusible safety switch and a separate fuse

Fusible Safety Switch


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

Switch Circuit Types and Terminology

1.Pole
The term pole refers to the number of circuits that can pass through a switch at one time.This is the number of circuits that the device can connect and disconnect.The following drawing, for example, shows a 3-pole safety switch.The three circuits are mechanically connected so that all three poles connect and disconnect the line and load simultaneously when the switch is operated. In this example, each pole is fused for overcurrent protection.
1
1

2.Throw
Throw is the term used to specify the number of circuits to which a conductor can be connected. All the examples shown so far have been for single throw switches. However, Siemens also offers double throw switches in both general duty non fusible and heavy duty fusible and non-fusible designs. Double throw switches are intended to transfer loads from one power source to another or to connect a single power source to either of two loads. For example, the illustration shown below shows 3-pole, non-fusible double throw switches. For either of the two applications, no power is applied to a load with the switch in the center (off) position, and only one set of contacts can be closed at a time.
1
In the application on the left, with the switch in the up position, the upper power source is connected to the load. With the switch in the down position, the lower power source is connected to the load.
In the application on the right, with the switch in the up position, the power source is connected to the upper load. With the switch in the down position, the power source is connected to the lower load.

Application

A safety switch can be used as a disconnecting means for a motor. National Electrical Code®
(NEC®) Article 430.102
requires a disconnecting means within sight of a motor and the machinery driven by the motor.The NEC® considers this to mean that the disconnecting device must be visible from the motor and machinery driven by the motor and not more than 15 meters (approximately 50 feet) away.
Regardless of where the safety switch is used, the function is to provide a means to connect and disconnect the load from its source of electrical power.
1

With power removed, someone can safely service the machinery without coming into contact with live electrical components or having the motor accidently start.