Elevator Safety System


In the topic " Basic Elevator Components - Part One" , I indicate that the basic elevator components are as follows:


  1. Car. 
  2. Hoistway. 
  3. Machine/drive system. 
  4. Safety system. 
  5. Control system. 
I explained the elevator car components in this prevouis topic, and I explained the Hoistway and its components in the topic "Basic Elevator Components - Part Two". also, I explained the  Machine/drive system in the topic" Elevator Machine and Drive System ".


today I will continue explaining the forth item from basic elevator components which is Safety system  as follows.
You can review the following prevouis topics for more information and good following. 



Forth: Safety system:

Layout of Elevator Safety System
The following list describes all the safety components used in electrical traction elevator safety system:

  1. Device for locking landing doors (Hoistway Door Interlock).
  2. Progressive safety gear.
  3. Overspeed governor.
  4. Buffers.
  5. Final Limit switches.
  6. Other safety devices and switches.


1- Device for locking landing doors (Hoistway Door Interlock)

Device for locking landing doors

  • It shall not be possible in normal operation to open the landing door (or any of the panels in the case of a multi-panel door) unless the car has stopped, or is on the point of stopping, in the unlocking zone of the door. 
  • The unlocking zone shall not extend more than 0.2 meter above and below the landing level.
  • The hoistway door locking mechanism provides a means to mechanically lock each hoistway door and the elevator cannot leave a landing unless the doors are fully closed and secured. 

Hoistway Door Interlock
  • They are also interconnected electrically to prevent operation of the elevator if any of the elevator’s hoistway doors are open. Should the doors be forced open, the interlock circuit will be broken, causing the elevator to immediately stop. 
  • Each landing door shall be provided with a locking device satisfying the previous conditions. This device shall be protected against deliberate misuse. 
  • Landing doors shall be capable of being unlocked from the outside with the aid of key , which fit the unlocking triangle (Hoistway Emergency Door Keys).

Hoistway Emergency Door Keys 

Hoistway Emergency Door Keys

Hoistway Emergency Door Keys permit the unlocking of the hoistway door interlock.

Escutcheon Tube

Escutcheon Tube

  • The keyhole on the upper portion of a hoistway door that accepts a hoistway emergency door key and permits unlocking of the hoistway door locking mechanism. 
  • These keyholes are usually located at the bottom and top floors, but may also be on other selected floors or all floors. 
  • You may find a lock covering these keyholes on some new elevator installations. Locate these keys during pre-fires. 



2- Progressive safety gear 

Progressive safety gear

  • Safety gear is a mechanical device for stopping the car (or counterweight) by gripping the guide rails in the event of car speed attaining a pre-determined value in a downward direction of travel, irrespective what the reason for the increase in speed may be. 
  • Progressive safety gear retardation is affected by a breaking action on the guide rails and for which special provisions are made so as to limit the forces on the car, counterweight o balancing weight to a permissible value. 
  • Pair of safety gears is mounted in the lower part of car sling and operated simultaneously by a linkage mechanism that actuated by overspeed governor. 

Safety Mechanism:

The progressive safety gear and the braking device are activated by means of a linkage with a shearing mechanism as shown in below image. 

Safety Mechanism

The release lever can be mounted on the right-hand or left-hand side, in front or at the rear dependent on the type of installation.


Operation of Safety Mechanism:
Dependent on the direction the safety lever is pulled upwards or downwards; the movement of the lever is transmitted to the shearing mechanism by means of a rocker. The grip wedges of progressive safety gear or braking device which are linked with the safety-gear levers are released from their rest position between rail and jaw body which is maintained by a spring assembly. The safety-gear lever assembly which is arranged in the form of a shearing mechanism ensures that the progressive safety gears and/or braking device are activated simultaneously and in pairs.

Reset:
The progressive safety gear and the braking device are reset by moving the car opposite to direction of safety gear operation. (Move car in electric recall mode, or if necessary, by releasing the car from the engaged position).


 

Safety switch is mounted on the bottom transom on the side of the safety-gear. The switch is operated by the movement of the safety-gear lever up or down according to actuation direction if the car travels at over speed. The switch interrupts the safety circuit causing machine drive power off.


3- Overspeed governor 

Overspeed governor
  • Overspeed governor function is to actuate the safety gear if the car speed exceeds 115% of itys rated value. 
  • Usually a cable is attached to the safeties on the underside of the car, called the governor rope. This rope runs down through a pulley at the bottom of the shaft and back up to the machine room and around the governor sheave. 
  • When over-speeding is detected, the governor grips the cable which applies the safeties that wedge against the guide rails and stops the car. 
  • The overspeed governor works on the floating principle with a cam curve and roller guided rocker. 
  • It is situated either in the machine room or in the head room. 
  • Overspeed governer is provided by a factory adjusted switch activated when the tripped speed is reached to disconnect the machine drive starting with governor pulley blocking. 


4- Buffers 


A Buffer is a device designed to stop a descending car or counterweight beyond its normal limit and to soften the force with which the elevator runs into the pit during an emergency. They may be of polyurethane or oil type in respect of the rated speed.

There are two principal types of buffers in existence:
A- Energy accumulation: accumulate the kinetic energy of the car or counterweight.

B- Energy dissipation: dissipate the kinetic energy of the car or counterweight.


Polyurethane buffers which are energy accumulation type with non-linear characteristics are used for our lifts that have rated speed not more than 1 m/sec.

Polyurethane buffers have three shapes as shown in the below image.


Polyurethane buffers Shapes

The main types of elevator buffers are: 

Main Types of Elevator Buffers

a- A Spring Buffer is one type of buffer most commonly found on hydraulic elevators or used for elevators with speeds less than 200 feet per minute. These devices are used to cushion the elevator and are most always located in the elevator pit.


b- An Oil Buffer is another type of buffer more commonly found on traction elevators with speeds higher than 200 feet per minute. This type of buffer uses a combination of oil and springs to cushion a descending car or counterweight and are most commonly located in the elevator pit, because of their location in the pit buffers have a tendency to be exposed to water and flooding. They require routine cleaning and painting to assure they maintain their proper performance specifications. Oil buffers also need there oil checked and changed if exposed to flooding.



5- Final Limit Switches 


  • Final limit switches shall be set to function as close as possible to the terminal floors (the highest or lowest landing of lifts), without risk of accident. 
  • Final limit switches shall operate before the car comes into contact with the buffers. The action of the final limit switches shall be maintained whilst the buffers are compressed. 
  • After the operation of final limit switches, the return to service of the lift cannot occur automatically. 


6- Other Safety Devices and Switches 


A- Overload Device 



Load weighing device or the overload sensor is mounted on the lower transom to snse the nearness of car floor during loading of acr isolation springs. The sensor is operated by altering the distance between car floor and sling dependent on the load. A distance screw shall be provided close to the sensor for protection purposes. Set the distance screw in such a way that it projects the sensor by a approximately 1 mm, so that the sensor is protected in the case of shock motions which raise during safety gear operation of the car.


B- Door Protective Device 

any type of device used with automatic power operated doors that detect obstructions to the normal closing of the elevator doors (though contact may occur) and either causes the doors to change the door motion by either stopping it, or causing it to reverse (reopen) or go into some other mode of operation, such as nudging. A safe edge, a safety astragal, a photoelectric device (safe ray), and electrostatic field device are examples of door protective devices.

Photo-electric and infrared sensors

Photo-electric and infrared sensors : A sensor between the hoistway and car doors that detects objects in their path and prevents the doors from closing. 

Safety edges: movable strips on edge of door that activates a switch to reopen if something contacts it


C- Emergency stop switch: 

The red switch inside some cars that cuts off the power to the car except for the lights, alarm and communication system.

D- Seismic switch/device 

Seismic switch/device

  • Seismic switch is A motion sensing device on some elevators installations. If it is activated the elevator will move away from the counter weights to the next landing with its doors open and inoperable. 
  • This device overrides phase (I) and phase (II) operation unless phase (II) operation is already in effect. 
  • If this device has been activated it can mean that an unsafe structural condition exists. 
  • This device is located in the machine room 


E- Emergency Alarm Switch:


It will sound an alarm when activated by a passenger and in most elevators; an emergency telephone or intercom can serve as a link to assistance if the car should stall.


F- Anti-Egress Lock Device 

Anti-Egress Lock Device

Allows car doors to open from inside by only 4 inches unless car is near landing.



G- Pit Safety Switch

Pit Safety Switch

This device, strategically located at elevator pit entry and exit points, greatly enhances safety for personnel who work in the elevator pit by preventing unexpected elevator movement.


H- Fire Fighter Services Devices/Switches:

It includes two types of switches as follows:

1- Phase I Switch 

Phase I Switch

  • A mode of operation activated by a smoke detector located in an elevator lobby, elevator machine room, elevator hoistway or by a keyed recall switch. 
  • This activation returns all cars to the main egress lobby or an alternated designated landing, opens the elevator doors and removes the cars from service. 
  • If the car is on independent service, the elevator will revert to Phase I recall in about 60 seconds. 
  • An emergency responder may want to manually activate an elevator recall because of an emergency situation that has not been detected by an automatically alarm system. 
  • Conversely, an emergency responder or maintenance person may want to override an automatically initiated recall because an alarm area has been found to be safe and the automatic system has not yet been reset or has malfunctioned 
  • The keyed recall switch has an “On”, “Off” and sometimes a “Bypass” position. The key is removable in the “On” and “Off” positions only. 

  1. On - Puts elevators into phase I recall. 
  2. Off - Puts elevator back into normal service. 
  3. Bypass – Put elevator back into service regardless of whether the smoke detectors are reset. The key must be kept in switch when it is in the bypass position. 

Note: The “bypass” feature is being replaced with a “reset” feature for elevators installed or altered under ASME A17.1a, 2002.



2- Phase II Switch 

Phase II Switch

  • A mode of operation activated by the firefighters independent service key switch inside the elevator. 
  • This can only be activated when a phase I recall of the elevators is in effect and with the elevator at the lobby or designated floor with the doors open. 
  • Phase II gives the emergency responder the option of accessing an active alarm area by elevator, but alters the functions of the elevator car in ways that increase the margin for safety of the responder. 
  • When Phase II is activated, the responder assumes manual control of an individual elevator car. The car works almost normally, with important exceptions. The firefighter can instruct the car to go to any floor it serves. When the car arrives at the floor, the doors do not open automatically. The doors will open only with continuous pressure on the door open button. If the button is released during opening, the doors will stop opening and reclose. This feature is meant to protect firefighters from opening the doors into a fire situation. Once the doors have been fully opened the elevator will stay in place with the doors open and they will remain open until a command is given to close them by continuously holding the door close button. The elevator will then remain in place with the doors closed until a floor button is pressed, when it will then start the same process on the selected floor. 
  • The Phase II keyed switch in the car has an “Off”, “On” and sometimes a “Hold” positions as follows: 
  1. The “Hold” position: allows fire personnel to remove the key and search a floor while the elevator car is waiting with the doors open. 
  2. Off position: Puts elevator back into phase I control, unless elevator is not in phase I, then the elevator will stay at that location with doors open until it is put into phase II again, returned to lobby and then switched to the off position. 
  3. On position: Puts the elevator into firefighter’s independent service. 


In the next Topic, I will continue explaining the Basic Elevator Components. So, please keep following. 


Note: these topics about elevators in this course EE-1: Beginner's electrical design course is an introduction only for beginners to know general basic information about elevators as a type of Power loads. But in other levels of our electrical design courses, we will show and explain in detail the Elevator Loads Estimation calculations.


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