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Safety and Protective Devices Used in an Electric Elevator
In this article, we will throw some light on the different safety and protective devices used in electric elevators.
Electric elevators are the electrically operated devices which are used for lifting up and down, moving load in suitable direction, and lifts in multistoried buildings for facilitating up and down movements of occupants. Therefore, it can be said that an electric elevator is an electrically operated device used for transporting at a fixed place.
Safety and Protective Devices Used in Elevators
The elevator safety device is the part of elevator which is used for protecting the elevator so it can run safely. The primary function of the elevator safety device is to stop the elevator car and to hold the elevator car at the elevator guide rail in an emergency. Hence, the elevator safety devices provide effective protection for elevator running in safe.
The safety and protective devices used in the elevators may be classified into following categories as −
- Electrical safety devices
- Mechanical safety devices
The principle safety devices (both electrical and mechanical) are as follows −
Guide grips and overspeed governor with governor switch
Car operating switch
Car safety switch
Terminal limit switches
Over travel limit switches
Slack cable switch
Door switches
Compensating rope sheave switch
Buffers and air cushions
Main line service switch and fuses
Circuit breaker and overload relay
Overspeed slowdown relay
Phase failure protective relay
Phase reversal protective relay
Guide Grips and Overspeed Governor
The guide grips and overspeed mechanism is mounted below the elevator car (cage) with a small winding drum which is connected to the overspeed governor by a steel rope. The overspeed governor is arranged with a grip so that if the speed of the elevator exceeds the normal speed, then it holds the governor rope and forces the grips against the guide rails so as the stop the elevator car.
Car Operating Switch
The car can be automatically set to different speeds using the car operating switch. If operator’s hands are removed from the lever of the car operating switch, then it returns to OFF position.
Car Safety Switch
The car safety switch is employed in electric elevators for the purpose of stopping the car in emergency in case of the failure of the car operating switch. Terminal Limit Switches Terminal limit switches are used for bringing the car smooth rest on landing.
Over Travel Limit Switches
The function of over travel limit switches is to stop the car in case of the failure of the regular terminal limit switches.
Slack Cable Switch
Generally, the slack cable switch is used on a drum type elevator to open the control circuit in case of slack rope, caused by the car or counter weight, being caught in the guides.
Door Safety Switches
The door safety switches in combination with the door locks prevent the elevator car operating unless all the doors are closed and locked.
Main Line Service Switch and Fuses
The main line service switch and fuses are used to open the elevator circuit in case of a fault like short circuit, etc. They are mounted in an accessible location in the elevator machine, room and usually in a metal cabinet.
Circuit Breaker and Overload Relay
The circuit breaker and overload relays are used to protect against the overloading of the elevator itself. They are normally set below the rating of fuse.
Overspeed Slowdown Relay
The overspeed slowdown relays are employed for automatic retardation of the elevator. The overspeeding of the elevator in either direction will cause the overspeed relay to operate and hence, automatically reduce the speed of the elevator.
Phase Failure Protective Relay
The failure of any phase causes the phase failure protective relay to open the controller circuit and hence disconnect the motor from the supply lines.
Phase Reversal Protective Relay
The phase reversal protective relay is used to provide protection against the reversal of phases, it immediately opens the controller circuit in case of phase reversal and prevent the elevator motor being connected to the lines until the relation of phases is corrected.
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