Have You Got The Right Relay?

Have You Got The Right Relay?

Bill Killick, Technical & Quality Manager at Finder, offers some advice on how its Interface Relays work, the part numbers involved and how to make sure you have the right one.

Finder Interface Relays are a combination of Relay (Electromechanical or Solid State), Base (Socket) and other optional components (such as timers, suppression modules etc.). They’re often used between a control system (e.g. PLC) and a higher power load (e.g. motor) because the control system can’t drive the load directly. Because they’re quite versatile, small sized and cost-effective they can be used in lots of other applications too.

It’s all about the base
The base of an interface relay has lots of functions; the most fundamental purpose is to allow easy connection to the relay – with either screw or screwless terminals – to mount it on a rail, and allow easy wiring between bases (jumper links). All Finder interface bases have these features.

Often the base also features electronics in it to modify the incoming control signal – for instance, changing a mains input to a lower DC voltage and then applying that to the relay coil. That’s why some combinations feature a 60VDC relay in a 230VAC interface. Some interfaces can even accept 24-240V AC or DC on the input (A1 & A2) even though the relay itself cannot; another function of some bases is to provide leakage current suppression which may help when PLCs are not turning off standard relays – it’s all being taken care of in the base.

What’s your number?
As a minimum, an interface relay is a combination of Relay & Base and each of these have their own part numbers (because Finder also sells these separately). When they’re combined they gain a new part number that’s often similar to the relay part number but not quite the same. Finder always prints the relay part number on the relay and the base part number on the base.

On some bases Finder also prints the interface part number. The best way to check the exact combination of part numbers is to use the latest list in the catalogue. At the time of writing the two main lists can be found at:

For 38 Series (towards the end)

For 39 Series (towards the end)

The picture below shows a 39 Series Interface Relay. At the top is the black relay with the relay part number Underneath the part number is a symbol for the coil, then 24V then a DC symbol – this is the input voltage to operate the relay coil (it is the ‘7.024’ bit of the part number). Next to that is ‘6A 250V’ and an AC symbol, then a contact symbol – these are the contact side ratings, including a 250VAC switching rating which is NOT the same as the input (coil) voltage.


On the left in the middle of the base is the base part number (marked ‘Socket’) This particular base shows other part numbers because it can be used to form other combinations of interface. It can be confusing so it’s best to just look at the ‘Socket’ number.

At the bottom of the base are various approval markings and these often come with ‘V’ and ‘A’ values. These are specifically to do with the approval tests and are NOT the same as the normal relay and base values.


The picture above shows an extract from the catalogue showing the Interface part number, the coil voltage and the relay and socket part numbers. This is how you can check you have the correct combination. In this example the relay and base combine to an Interface of You can also see that the 230VAC interface uses a 60VDC relay.

For more information about the range of Interface Relays available from Finder visit: www.findernet.com

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