Why would anyone specify a dry contact?

Hello all, this is my first post!

I am a mechanical engineer with background in electronic packaging. I work in the water industry and there is a large need for water meters to send an electrical signal to an external device (usually PLC occasionally SCADA) to trigger an external valve, activate a pump, perform SOME type of mechanical action. This is usually required for some process--i.e. injecting chemicals into the stream (depending on the type and use of flow of course).

The issue I am having is--why would the owner/operator of the third party device specify a dry contact requirement? I see the value in the dry contact if there is no form of external power that can be applied to the device sending the electrical signal, but other than that I am struggling to see value.

Is there a safety issue when a "wetted" contact is used?
Will some PLC simply not function unless there is a dry contact?? (I would struggle to see how this can be--maybe there is a limitation on the frequency of the incoming electrical signal, that I can grasp).
Perhaps it is a way to protect the 3rd party's equipment--they want it on an isolated circuit in case the device sending the electrical signal shorted or had an issue...?

Oh ye electrical engineers! Please help my feeble mechanical mind...

Thanks!

Hand

Welcome to the Forum.

A dry contact is specified because you cannot always be sure that the "wet" voltage from the device is going to be compatible with whatever it is signalling. Not just the voltage level itself, but the potential for introducing electrical noise is often a concern with sensitive devices, such as PLC inputs.

We always specify dry contacts because we use generic PLC's and have no idea which contacts will be used for what. Dry allows AC, DC, different voltages, etc. Saves having to use interposing relays to match solid state output specs. Much more versatile.

Dry contact just means it gets powered from whatever it is connected to.

e.g.- if it is going to a scada system input, whatever power is being used to interrogate the contact with comes from the scada system.

A_Large_Hand said:

Hello all, this is my first post!

I am a mechanical engineer with background in electronic packaging. I work in the water industry and there is a large need for water meters to send an electrical signal to an external device (usually PLC occasionally SCADA) to trigger an external valve, activate a pump, perform SOME type of mechanical action. This is usually required for some process--i.e. injecting chemicals into the stream (depending on the type and use of flow of course).


Hand

Click to expand...

I read your post again and maybe I misunderstood. Are you talking about a water meter output sending a signal to another device input? If, for example, you want to send a command contact to a PLC input from the water meter output, the PLC only requires a contact closure connected between the PLC common and input terminal. The PLC would normally supply contact whetting voltage, such as 12V or 24V DC. Ditto for a SCADA input. You would need a "dry contact" on the water meter. If the water meter has a solid state output, it may not be compatible with some external devices. If that's not an option, you'll have to use an interposing relay to provide the dry contact. PLC's and SCADA systems I'm familiar with use a contact closure to supply a digital input.

As others have said you want control power for a given system from the same source
for several reasons
common frame potential, important with ddc/electronic systems
isolation/disconnection, remove power for maintenance
big surpise you turn off control system and a seperately sourced nc contact is hot from another source

meternerd said:

I read your post again and maybe I misunderstood. Are you talking about a water meter output sending a signal to another device input? If, for example, you want to send a command contact to a PLC input from the water meter output, the PLC only requires a contact closure connected between the PLC common and input terminal. The PLC would normally supply contact whetting voltage, such as 12V or 24V DC. Ditto for a SCADA input. You would need a "dry contact" on the water meter. If the water meter has a solid state output, it may not be compatible with some external devices. If that's not an option, you'll have to use an interposing relay to provide the dry contact. PLC's and SCADA systems I'm familiar with use a contact closure to supply a digital input.

Click to expand...

Yes, I believe this would be an exact setup. In this sense the meter is really just acting like a giant "switch" that will close at the appropriate time to allow the PLC to be continuous through the now closed "switch".

Thanks for all of the great feedback, really helps my understanding. Great forum, I'm looking forward to using it--I also like working on Arduino projects so I'm sure I'll dig up lots of useful topics here.

Hand