safety-edge-types

    Safety edge types

Safety edges are one of two pressure sensing safety techniques.

The characteristics need to be chosen to match the application.

We look at the options.

Author  :    Huw Jones

See also

Why and when to use an edge

Unlike photobeams, safety edges only work when you make contact, and then you are committed. So, why use safety edges or other contact only protection? 

Like photobeams, edges are a visible endorsement of the gate's safety, so more often used on a school pedestrian gate than a heritage wrought iron gate. Rubber edges also provide a soft buffer.

A horizontal photobeam is only useful at one height, so may not fully mittigate a risk. A safety edge is active along it's length so is better targeted to risk of shear which is particularly common on sliding gates.

Unlike photobeams, safety edges only work when you make contact, and then you are committed. So, why use safety edges or other contact only protection? 

Like photobeams, edges are a visible endorsement of the gate's safety, so more often used on a school pedestrian gate than a heritage wrought iron gate. Rubber edges also provide a soft buffer.

A safety edge is active along it's length so is better targeted to risk of shear. A horizontal photobeam is only useful at one height, so may not fully mittigate a risk.

Taught wire

Taught wire safety edges are large rubber edges suitable for fast closing gaps. They have good mechanical protection and trigger from pressure in three directions.

A wire runs the full length under tension. When it is pressed, it triggers a micro-switch inside.

Pros & cons

The micro-switches are built into the end, so that part of the edge is not sensitive to pressure.

There is normally a second micro-switch to detect wire failure, so are Cat1 or Cat3.

They are simple, reliable, cheap, but a bit bulky. Mechanical protection is good.

Conductive rubber

A rubber tube has two semi-conductive strips inside. They connect when pressure is applied. They come in all sizes. Larger profiles also respond to side pressure.

The end of the edge is terminated with a resistor. The output reads 8k2, or zero when pressed.  They must be connected to a suitable 8k2 input, or connected via an 8k2 processor.

Pros & cons

Conductive rubber edges are sensitive over the full length and at a wide angle. Inputs are continually monitored (Cat3) contact.

Several edges can be chained together on one 8k2 input, but cables in the chain are sensitive to poor connection.

Piano edges

Correct name; strip hinge edges, are formed between two aluminium strips and two or more microswitches. 

Generally, these devices have small activation distance and small overtravel, so good for only slower closing gaps, and they only activate in one direction. 

Pros & cons

Two microswitches give the redundancy required for Cat1. Any number of edge contacts can be wired in series, but would not be Cat2 tested.

Piano edges are low profile and discrete, so popular with clients.

Other technologies

Where there is a market there will be odd technologies and legacy products. These all use soft rubber tubing.

Pneumatic. Road users will have seen the temporary rubber tubes pinned across the road for counting. Similar pneumatic edges were used in gate automation. They failed when the rubber perished.

Infrared edges used a photobeam down a rubber edge. As the edge is pressed, the beam is blocked, so activating an output. It worked well and some had adjustable sensitivity, but was not widely adopted.

Ball and cup edges were a mad expensive idea adopted by German gate makers. Alternate conductive balls and cups are strung on a cord like beads. They conduct when aligned, but not when mis-aligned. 

Wired 8k2 processors

Many control panels have 8k2 inputs, and some photobeams have an 8k2 input for conductive rubber edges.

If they are not available, or you need an extra 8k2 input, you will need a separate 8k2 processor. They are usually a module like the photo below, but can be small enough to fit inside the rubber edge.

Pros & cons

The modules often have DIP settings that allow two or more 8k2 edges to connect to the same input. The setting changes the input to 4k1 (half) or 16k4 (double). But as more edges are combined on a single input there are more sources of error and unreliability.

Wired 8k2 processor

Infrared beam

Infrared transmission systems are a version of a battery photobeam. IR works well on sliding gates.

The receiver is fixed near the motor. The safety edge wires to the battery transmitter on the gate.

Pros & cons

Some installers distrust radio safety. IR problems are easy to diagnose. Infrared transmission is cheaper than radio, but needs alignment.

Radio transmission

Radio wireless systems are becoming more common as prices come down. Battery life can be longer than on IR beam systems. 

The safety edge wires to a transmitter which can be fitted into large section rubber edges. 

Pros & cons

Radio signals are long range and do not need alignment. There can be a conflict between safety radio signals and remote controls. Be sure they are on a separate channel. Radio system problems are difficult to diagnose.

Sliding rail

Although not wireless, these systems are still available. A horizontal rail is fixed to the gate. The rail contains a sliding contact or spring coiled cable. The receiver inside the rail is fitted to the top of the ground post, which wires back to the controller. 

Pros & cons

Modern wireless systems have made these systems obsolete. Designed when batteries were unreliable and big. They were always very expensive.

Inductive transmission

Inductive transmission systems have a wire stretched along the length of the gate, and a receiver coil on a post. 

A current is induced in the steel wire when the edge is safe. When the edge breaks the induced current, the coil receiver senses a load loss, triggering the gate to reverse. 

Pros & cons

Better for short gates. The alignment is an additional complexity. Creates an additional shear edge at the receiver post. One channel per induction wire. No batteries required.

Edge response times

Closure gap speed, is the constant on a sliding gate because all parts move together. On a swing gate, there are gaps that close slowly near the hinge, while the end of the swing gate leaf moves quickly towards a wall or object in its path.

Activation distance of a safety edge is how far it needs to be pushed before the contact changes state. 

Overtravel  is the further compression of the edge after the activation distance. Overtravel ends when the edge is fully compressed.

The gate leaf will not stop as soon as the control panel receives the unsafe signal. The electronics is fast, but the motor will carry on spinning before it stops. Some motor stop quicker if they have an electronic brake.

Gate momentum (speed x weight) and hinge backlash are a store of destructive energy that will carry the gate further. The heavier the gate, the more dynamic energy it has.

The sum of activation distance and overtravel should be enough to decelerate the gate and absorb all the gate's dynamic energy. This is most important for the higher speed edges. 

In conclusion, the gate system has response times, kinetic energy, and safety edge energy absorbtion that cannot be calculated by mere installers. If this is the route to safety, you will need to test the force dynamically according to EN12445 because every gate is different. Installers are responsible for a safety compliance. 

Conclusions

• Transmission systems are down to preference. Infrared transmission are a good compromise of reliability, easy of fitting, and fault finding.
• Thick safety edges maybe unsightly, but are necessary for fast moving edges. Mechanical edges are better for slowly closing gaps.
• If gate safety relies on limiting force after it has come into contact with an object (use of safety edges as primary protection), you will need to prove safety by force testing

New developments

• Combined safety edges and radio transmitters are becoming very competitive