Technical Article

# What Are the Key Benefits of Mechanical Relays?

April 02, 2020 by David Peterson

## This article explores some of the key benefits and drawbacks of mechanical relays.

The relay is a component that has driven industrial devices and shaped the structure of electrical control systems since the very beginning of industrial electrical control. Newer solid-state relays have replaced the older mechanical versions for controlling some load devices, but there are many situations in which the mechanical relay is still superior.

### What Is the Function of Relay?

The relay is one of the most common components to be found within any industrial control cabinet. These small and inexpensive devices perform several functions, most of which include arrangements of sequential logic and isolation of operators from dangerous voltage levels.

The basic construction of a relay consists of an input coil with many turns of fine wire. The coil voltage may be low voltage DC such as 5, 12, or 24 volts, or it may require voltages of AC, either 24 volts or up to 120 or 230 volts. This voltage will always be printed on the case of every relay.

Application of the control voltage not only generates a magnetic field when energized but also provides a significant resistance due to the wire length. Therefore coils limit their own current but they also exhibit properties of any inductor.

The output contact sets consist usually of an input ‘common’ terminal, which connects directly to a normally closed terminal when de-energized, and then switches over the normally open terminal when energized. The output contacts, since they are simple switches, are tolerant of either DC or AC voltage, however, every relay will have a maximum voltage and current allowance.

A newer variation of a relay that uses solid-state construction has replaced its mechanical counterparts in some situations, but the mechanical relay holds distinct advantages in many cases.

##### This mechanical relay shows 4 contact sets displayed by the visible copper tabs. Each contact set has an NO and NC contact. This relay is ideal for logic control circuits.

Mechanical relays, unlike their solid-state counterparts, can contain multiple contact sets, as many as four or even more in a few cases. SSR’s are limited to a single set, since they are designed to simply activate a single load device, never for control logic sequences.

Another side benefit of these multiple contacts is that load devices with different operating voltages can be controlled with the same relay. For example, a single switch is meant to control a 24-volt indicator light, as well as a 110-volt motor - a single DPDT relay can handle this task with ease.

In most cases, each set of contacts in a mechanical relay contains both a Normally Open and Closed contact. SSR’s only have a single Normally Open contact, since again, the job is to activate a load device when energized - never to deactivate the load.

For both logic as well as load operation, it is often critical to determine when an input circuit is off, such as a motor power indicator. A light may be illuminated when the relay is de-energized, indicating the relay is off. This scenario is impossible for the solid-state relay.

Occasionally, relay contact sets will be “Single Throw” which means that they only contain the Normally open OR closed contact, but not both. Be aware of this when purchasing relays, and always check datasheets for specific part numbers before assuming that all mechanical relays are created alike - they are not!

##### A 4 Pole, Single Throw (4PST) relay which includes 2x Normally Open, and 2x Normally Closed contact sets.

In a mechanical relay, the contacts have lower resistance when turned on, usually around 0.05Ω. This would mean that, for example, a load circuit carrying 10 amps would only lose about 0.5 volts, and dissipate about 5 watts across the relay contact. Although any inefficiency is undesirable, that value is inconsequential in this high power circuit.

In comparison, some SSR’s have on-state resistance values comparable or even less than mechanical relays. However, some of them may have up to 10x this value, as much as 0.5Ω of resistance. This would lead to a voltage loss of 5 volts and a power loss of 50 watts in a similar 10 amp circuit. Because of this power dissipation, the backside of a general solid-state relay is metal, meant to be mounted to a heatsink to remove the extra energy.

##### A DPDT relay set in a DIN rail mounting socket, providing easy access to wire connections for the coil and contact terminals.

A fourth major advantage that mechanical relays hold is the ability to, in some cases, test them easily while still in the circuit. Troubleshooting usually requires isolation or removal of the component, and a bench setup to verify operation.

Many models of relay have a switch on the top that allows the technician to switch the contacts on and off, even with no power to the coil. This can isolate problems to either the input or output side instantly, never requiring a single wire to be removed.

##### A DPDT relay which includes a test switch at the top, allowing manual toggling between the Normally Open and Normally Closed contacts, which can be seen through the transparent window.

Even without the integrated test switch, many industrial models mount directly into a DIN rail socket, allowing for easy removal, test, and the replacement still without wire removal. Changing wires can lead to other problems including improper torquing and stripping of screw terminals, so it’s best when troubleshooting can remove the need for adjusting screw terminals.

### Preferable For Logical Control Operations

Finally, another benefit to mechanical relays is the lower relative cost. Although energy savings are desirable, the cost difference between devices can be upwards of $10 or$20, with the solid-state relays costing more. The decision must be made to choose one device over the other, and the cost is never overlooked.

Mechanical relays, although not the only option for switching loads, are usually preferred in the case of logical control operations, and when general, low-cost switching is desired to run a load device.

Do you prefer mechanical relays?