Hello,

Recently our company acquired an (older) power-plant. It turns out that the former owners had an extensive automated installation based on 110VDC. Just months before the take-over they changed most of their installation from 110VDC to 24VDC. Now all the contacts(level-switches, pressure-switches,valve-positionners,...) which previously for the past 20years were wired into 110VDC-circuits, are now wired into 24VDC-inputcircuits for a mainframe.
I expect that contacts that have been used for many years at 110VDC may have a certain amount of burn-in, making their usage at 24VDC a problem. Some people at the company advise me to change them all to gold-plated contacts ?!
Who has experience in these matters ?

 

If the contacts are rated at 110VDC but used at 24VDC, then that in it self does not pose a problem.

Actually, it is the load which is the important factor. As all of these contacts are used for inputs in DCS or PLC system, then the load is very little (because the impedance on the input is very high). Both 110VDC and 24VDC inputs have a relatively high impedance.

 

Reed switches will either work or not and do not have a fatigued stage, so to speak. The contact points are coated with an extremely thin layer of material like Rhodium today. If that layer is burnt the switches will fail - up until that point you should be OK if certain things were changed. At this time I should say - it’s your call; I’m just giving an opinion. Voltage, in itself, would not be the problem.

You may have a problem depending on the type of control circuitry they used. For example, if they used relays, you should have a snubber circuit. The little circuit is used to prevent any induction or arc back to the switch from a contact type relay. On an AC circuit it is a resistor/capacitor circuit on DC you would use a diode. Please see the link to “how to install a snubber circuit”. If the reeds do not see any arc induced controls you may be OK. A reed switch improperly connected may either fuse immediately or show signs of sticking where at tap on the tube will change the state. At that point it needs to repaired or replaced.

The page also explains when you need to use a snubber and when it’s OK not to.

http://www.almegcontrols.com/switch_specs.htm

If you have any questions contact Almeg Controls from the link.

Thanks
Bob Hogg

 

Normally a dry contact in a device is rated for a maximun voltage. If the contacts have been used for 100VAC you will have no trouble using them for 24VDC. I consider the most important thing now is to evaluate the actual state of each contact to define if you must change it or not.

Luck

 

hi

it is good that u will change to gold plated contacts because on 24v small resistance in contacts due to wear and tear. dust plays an important role also signals are often in milli or micro AMP (if u are using solid state sensors).

also one thing i will tell u when u go to 24 v also take in mind of cotact load (amp) rating as now u have to choose heavy cotacts.

 

What I call plating may be an issue when you switch to DC. When this occurs the field contacts will make less reliable contact closures. It is more common in wet or outdoor enviroments.

Go ahead with the project, just be aware that this issue could show up.

 

think of the various lever switches and such as the set of dry contacts which will close to provide an input to your mainframe or PLC device, most likely the digital inputs of this unit are already protected to prevent some inductive feed back, and most assuredly will not arc or spark, in fact they are probably rated at a maximum input voltage of 32vdc or 48vdc. The contacts on the end devcies, switches, ect, will probably not have very heavy loads on them, these are control circuits, which means that your not switching 10's of amps, if they are you will need interposing relays to prevent all of that current from blowing out the digital inputs to the mainframe or PLC.

Changing the contacts to gold plated will only reduce the relatively small amount of resistance acroos the contacts, if the various contacts are indeed burent, dusty, then either clean them or replace them. It was not uncommon for these systems to wrok for 80+ years without failures - in fact I did a remodel of a 100 year old power plant in Boulder, CO. same deal - all 110vdc control, we used interposing relays to provide the interface to our PLC's 24vdc rated digital inputs and did the same for the outputs - just good policy, don't bring high voltages or foreign voltages to your PLC inputs / outputs.

In fact our SCADA implemetation made this plant viable for the next 20+ years as a completely remote controlled hydro power plant - it was awesome, this place was like a museum.

Any ways, more questions?
contact me,

matt hyatt
technical consultants
[email protected]