Isolating relays on PLC output?

Hi all,

Is it common practice to use an isolating relay between each PLC-output and its corresponding device? What are the pros/cons?

Thanks for your input

Dave

I think it's pretty common practice. Besides isolation, a couple of the 'pros' that come to mind are:

The relays are easily replaced, unlike the ones soldered in the PLC. Normally, you would choose transistor outputs in the PLC to drive the isolation relays. Since transistors have no moving parts, they should last indefinitely.

You can have different voltages on individual outputs. PLCs often have multiple outputs on one common, which all switch the same voltage.

The only real 'con' I can think of is the added cost.



-Eric

Eric Nelson said:

I think it's pretty common practice. Besides isolation, a couple of the 'pros' that come to mind are:

The relays are easily replaced, unlike the ones soldered in the PLC. Normally, you would choose transistor outputs in the PLC to drive the isolation relays. Since transistors have no moving parts, they should last indefinitely.

You can have different voltages on individual outputs. PLCs often have multiple outputs on one common, which all switch the same voltage.

The only real 'con' I can think of is the added cost.



-Eric

Click to expand...

Ditto,Ditto. I always use interposing relays.

It is a common practice, but you should consider the required switching rate. If the output is required to switch very fast, a relay may not be able to physically meet the switching rate. Also, higher switching rates means that the electro-mechanical relay will wear out sooner. If a high switching rate is required, you should look into a solid state relay.

Regards

Hello,

One caveat to be aware of is the number of times the output will toggle on and off in the expected lifetime of the output.

For example, an output that is used as a watchdog to another system and will toggle once a second or so, would NOT be a candidate for any mechanical device - on or off board relay.

Even a relay that had an expected life of 10,000,000 cycles would expire very soon.

It also depends on the load, for example if you're switching high inductive loads then it's almost imperative that you use an isolating relay to protect your output transistors or relay.

If you're not expecting to switch high current loads then I would say it's not necessary but I'd always use a terminal fuse to protect on short circuit.

James

I always put indicator lights/LED's direct to the plc output. Solenoids and others that's a different matter, depends on the load and what it is driving. Regards Badger.

Modern PLC Output Cards are fairly well protected out of the box. So, most standard loads can be driven directly. However, over the years, the rating of each Output Point, and the Output Card have dropped some with the smaller foot prints. Typically, an Solid State Output Point may only be rated for 0.5A, and the Output Card rated for 8A. For a Relay Output Card 2A per point, and 16A for the card.

I drive Pilot Lamps, Relays, small Contactors, and small Solenoid Valves directly. I use Interposing Relays for larger Starters and Solenoids, mixed Voltages, and between different machines and PLCs.

The down side of Relays is limited life due to mechanical wear. The advantage is easy replacement, even in PLC Output Cards, many have plug in relays.

As mentioned, Relays are suitable for heavier loads that are on most of the time. If you have a high cycle rate, the Relay will not last long. Reasonable on/off switching rates can be handled by up sizing the Relay.

Solid State Outputs should last a long time if they are not abused, and stand up to high switching rates.

Stu...

Horses for courses - and all that has been said is relevant.

However, if we talk about A-B ControlLogix, there is a family of I/O modules with "Diagnostic" features. The output modules can detect "No Load", and have "Electronic Fusing".

If you were using these modules, then an isolation relay negates the benefits of using them, since you are only detecting that the relay is connected, not the device you want to power.

Like many others I use relays on most outputs, excepting pilot lights and so on. I use relays that have latching manual over rides on them. This is a marvelous troubleshooting aid when you are on a job site alone. It is also very helpful in testing logic etc. to be able to pull the relay and not have the load cycling on and off every time you cycle the output.

We used to have 32-channel 24V output modules with relays on each output.
However, nowadays we use the fine-grained "slice" kind of i/o modules.
Kind of the output relay and the PLC output being one and same.
It saves a lot of space and wiring --> cost.

I dont like the relay output modules with 8, 16 or 32 channels on one card. Too inflexible, cumbersome wiring, and if one output goes you have to replace the whole card which may be a real problem on a running system.

I use to put a relay for every thing out of my panel .if it high rate I use SSR.
In the panel I use to put relay only for high consumption component.
Inside panel I use 1.4 W mini contactors.for large size contactors I use intermediate relay .

Everything that has been said is true and good advice, except...

I only use interposing relays when the load is too great for the relays on the output module. I like to eliminate wiring, and extra points of failure (which the extra wiring itself quite often is - as well as the extra relay).

The other situation is when number of cycles will be too great for a relay and then I use an interposing SSR connected to a transister output.

I'm sure that there are folks who have a larger installed base than I do, but I have thousands of points installed this way and I can't remember the last time I had any problem.

General rule of thumb is to minimize use of interposing relays.

However, sometimes you have to use them.

Other things to consider are the heat, load (resistive or inductive, and the amount of space used. Of course there are the Phoenix type SSRs that are the size of a terminal strip. Then you may need an interposing relay for the interposing relay.