Interlocking devices are safety features designed to lock people out of electronic devices while they are energized by isolating power before access can be gained. A simple example of an interlocking device is a washing machine that stops agitating when the lid is opened.
In situations where back-up generators are used, an interlocking device consists of one or more switches that prevent the simultaneous operation of a generator and the main power source. They cannot operate at the same time because doing so could result in an overload of the system(s) and potentially create a hazardous situation for utility employees working on the main power feed.
Interlocking devices prevent operation of both power sources at the same time and allow circuit breakers to operate without interference if an overload occurs.
Most interlocking devices consist of some kind of mechanical device designed to manage the movement of electrical circuit breakers and some allow the use of locks to prevent accidental or unauthorized activation of the main power system.
There are two types of electrical interlocking devices, the first of which is “power interlocking,” where the power source is simply interrupted by the opening of a “guard” that can only be opened using a key. Once the key is turned to unlock the “guard,” it is trapped in that position and cannot be moved until the guard is closed and locked, making it impossible to open the guard without isolating the source of power and, likewise, making it impossible to turn on the power without first closing and locking the guard.
The most common type of power interlocking system is one that uses a “trapped key,” whereby a key is literally trapped in a certain position while the switch is “on.” Turning the key causes the contacts on the isolation switch to be locked in an open position, thereby isolating the supply of power, and enabling withdrawal of the key.
The trapped key system does not require electrical wiring to the guard and has proven to be very reliable. The biggest disadvantage to this type of system is that the transfer key has to be used every time, making it impractical if more than one person frequently requires access. Personnel keys are recommended for situations where more than one person accesses the guard frequently and using a personnel key ensures that nobody will be inadvertently locked in the guarded area.
In a trapped key system, the lock can only be opened by the designated key, which makes it impossible to break into the system’s lock using screwdrivers or other tools. Excessive force applied to the key will only result in a broken key, not a broken lock. In situations where there is more than one lock, key codes are used to prevent unauthorized access.
The second type of electronic interlocking device is called a “control interlocking,” where switching the current that controls the device interrupts the power source. This is the most commonly used interlocking system and operates by detecting movement and opening the contacts on the switch whenever the guard is not completely closed.
Interlocking devices provide a means of preventing accidental employment of two sources of power at the same time, which could result in electrical overloads and personal injury or death to those working on the power system.
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