I am currently involved in a project upgrading a control system on a plant built in the 1970s. Part of the project involves converting panel annunciators (i.e. panel mounted lights indicating running status of a pump motor) over to a modern conventional PLC/DCS.

Originally the relay contacts and lights operated on 110VDC but the new system will be using 24VDC for loop power. The input impedance of the new system is quite high ~4kohm so the loop current through the relay contacts will be ~6mA. The loop power will therefore be decreasing from a small lamp (estimated 1 or 2 Watts at 110VDC) down to 0.15 Watts after the upgrade.

It has been brought our attention there may be a problem get the relay contacts to make a "solid" conductor when the contact closes. The reason for this is the minimum power required to make good electrical contact through any oxide coating or other poor conducting material that may have formed over the years. Unfortunately I don't have the current relay specification to hand at the moment.

I have two questions:
Is this a genuine problem? How many other people have encountered problems when converting circuits with relay contacts from high power down to low power?

I have read the thread "Oxidation of Relay Contacts", which gives some insight, but am wondering if there other personal experiences that are out there. Any help appreciated.

 

You betch'a, been there, done that, and have the T-shirt (and scars) to prove it.

Using a 2 Amp rated relay contact to switch ma of current is a sure recipe for future problems.

You can contact me at jmiille at miille dot com if you would like some more details.

Jerry Miille

 

Andray Bath asked about minimum current - "Is this a genuine problem?"

Yes this is a genuine problem. I have seen this on relay contacts that were converted from 120VAC to 24VDC and going into PLC inputs. There is no clear answer though as to how much of a problem it will be. It depends upon the contact material and the condition of the contacts. I have even seen it on relatively new relays which were only ever run on 24VDC, but had the wrong contact material.

This is the sort of problem that may not show up until several weeks after installation. It is also difficult to troubleshoot as it can be intermittent. In other words, brief initial testing of the relay contacts may not be indicative of long term performance. If reliable operation is important, it is probably best to simply replace them all. Use relays with a low (or no) minimum current rating.

If the relays date back to the 1970s, it is also a good opportunity to replace them with something more readily available today. Be sure to use a relay that plugs into a separate base. If these are simply interposing relays to convert between different voltages, consider using opto-isolators. These seem to be more reliable than small relays as they do not have a mechanical life limit. They are however more expensive than relays.

 

Michael

It is never a problem -- its an opportunity. I have used 24 volt contacts for DIs in many water treatment plants with success for over 20 years. When using 24 volt wetted contacts in harsh environments -- sewage plants -- this causes deterioration of the conductivity of the contact material -- due to H2S etc-- and other plating compounds. In the harsh environment areas I recommend 120 volt inputs which tend to burn through the oxide layer on the contacts during activation resulting in a definite contact closure being reported back to SCADA. In my experience -- the opportunity -- shows up years after installation and puts egg on you're face for future jobs with this client.

Dennis

 

What you are talking about can be a real problem if the currents (and voltage) are small enough and the contacts are made of a silver or copper alloy. The minimum current for a switch or relay is called the "wetting current." Silver or copper alloy contacts can oxidize to form a high resistance barrier to current, which can cause a contact malfunction. This is not as common a problem for higher voltages such as 120 VAC or 110 VDC, but can be a problem for lower voltages. The most common fix is to use gold plated, sealed, or wetted contacts.

I'm not sure if you will have a problem if you have 6 ma current, but you will have to check to see what the relay or switch wetting current specification are. My recollection in a past life with 110 VDC based annunciators was that they did not draw large amount of currents (low mA). However, if anything, the 110 VDC annunciators did provide a sufficient voltage driving force to burn off any oxide buildup.

A good discussion of switch contacts including wetting current can be found at:
http://www.allaboutcircuits.com/vol_4/chpt_4/2.html<http://www.allaboutcircuits.com/vol_4/chpt_4/2.html>

William (Bill) L. Mostia, Jr.
The opinions expressed here is mine and mine alone and are provided under the basis of "Caveat Emptor."

 

If your modernising the 1970's system, why not do away with the relays?...most DCS/PLC systems I have worked with will turn on a light. Or is it possible to turn on the indicator from the AUX contact on the motor starter/contactor. Relays are also available in solid state configurations that do not get corrosion problems.