Relays, Security’s Little Control Freaks

I once boldly heard a veteran technician proclaim, “Give me a relay and I can control the world!” That statement illustrates that tech’s personal confidence in both the use and power of the electromechanical device known as the relay. 

It has often surprised me how few techs in the low-voltage world understand and properly use relays. Since it has been a few years since we have directly touched on this subject, I thought it was a good time to provide a review for the older techs and orientate the new guys.

There Are 6 Popular Relay Types
The action of a relay provides for the control of a large variety of security and automation devices. A popular feature is the electrical isolation a relay can provide, such as controlling both low and high voltage from one control point. Another feature is controlling multiple devices in multiple systems, such as one relay controlling a low-voltage indicator (e.g. an LED) while simultaneously activating an overhead door motor and also shunting an alarm point.

As we have seen, a properly selected and applied relay can control many electrical devices in many applications. Let’s take a look at the many different types of relays:

Latching — This one has been around for a long time in the security industry and was actually the basis for earlier presemiconductor alarm panels. When an alarm was activated, the bell would sound continuously. When momentarily energized, this relay will toggle and mechanically stay in either the “on” or “off” position until reactivated. One of the advantages of this type of relay is that its internal mechanical control holds in either position without requiring continuous power to maintain it. This could be a plus in a wireless alarm control condition. Other types of relays can be wired to act like a latching relay, but steady power is needed to maintain their latched state.

Wet — This is referring to a relay in which the switching contacts are mercury wetted. The mercury coating on the relay contacts allows for better switching of low voltage (<1V) circuits. This type of relay often gets confused with the term “wet contact,” which can refer to a relay in which one of the switching contacts is always energized.

Reed — This is a relay in which the contacts are inside a hermetically sealed capsule. This can protect the contacts from corrosion due to atmospheric elements. It also will isolate any sparking from igniting in a potentially explosive environment. Caution should be taken not to exceed the current and voltage ratings of these more delicate relay contacts.

Contactor — This is probably one of the most popular types of relays used for alarm and automation control. This relay is designed for switching heavier electrical loads than in the circuit driving for the relay coil. The circuit being switched is typically separate of other circuits. This type of relay is often referred to as a “dry contact” in that both relay contacts are not energized continuously.

Solid state — This relay uses electronic components such as silicon-controlled rectifiers (SCRs) to switch devices. Even though more expensive, they can be more reliable than electromechanical relays that can be affected by environmental conditions such as humidity, air pollutants and vibration. Again, close attention needs to be made to the operating specifications of the electronic relay.

Time delay — Another popular type of relay in that it will either make or break a circuit within a planned period of time. This relay can either have an internal thermostatic controlled delay relay contact or a small electronic delay assembly with relay output. The former relay type will have a fixed time period in which the internal element will heat up and switch the circuit. The latter is popular and can be electronically adjusted, typically by a resistor selection, to switch within a programmed time period. This relay type can be used in security to allow a person to clear an area before a door shunt is reset.

Configuration Decides Capabilities
Now that we have an idea of the many types of relays, it is important for the technician to understand in what configurations a relay can be ordered. To do this we must understand the basic concepts of poles and throws.

One should remember that a relay is nothing more than an electromechanical switch. What we mean by that is an electrical impulse (on/off circuit) will energize a coil in the relay, which in turn will create a momentary magnetic field that will influence the mechanical movement and direction (throw) of multiple combinations of switch contacts (poles). 

Another way to look at a relay is the same way we look at a light switch. Many of us have seen a one-way light switch that will either switch a light on or off. This would be the equivalent of a single throw set of relay contacts. A two-way light switch configuration has a common dual-action contact that switches the circuit one way or the other. To do this with a relay, we would use a double-throw contact configuration.

Once this basic concept is understood, the sky is the limit. Relays can be ordered with anything from a simple single pole, single throw (SPST) to configurations such as a triple pole, double throw (3PDT).

Some may have heard the relay terminology normally closed (NC) for relay contacts. A normally closed set of alarm contacts means they are closed when the relay has no electrical energy to the relay coil. The opposite is true for a pair of normally open (NO) contacts.

In a double-throw (DT) configuration, you will have one set of NC and one set of NO contacts. Remember, you can take advantage of this where a NO contact will provide shunting an alarm zone and another NC contact in the same relay will provide a supervisory open condition so the control cannot be armed with a shunt left in place. This would be similar to the supervised shunt switch action in the January 2002 “Tech Talk” column.

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