Using lesser-rated contacts with 125Vdc digital inputs

[trat1208]

Hi,

Had a question about contact ratings related to a simple circuit . . .

In a new substation being built I have a SEL-2506 I/O module with 125Vdc inputs for wiring in miscellaneous building alarm contacts (125Vdc battery charger alarm, HVAC alarm, etc). Additionally, an H2 detector is being installed above the battery with an alarm contact that I'd like to wire up, but the manf literature says the alarm contact is rated for 0.5A@28Vdc and 0.5A@125Vac. So my contact does not have a 125Vdc rating and my DI's must be wetted with 125Vdc battery voltage (SEL-2506 does not allow the flexibility of wetting the DI's with AC or DC). The SEL 125Vdc digital input draws 4ma.

My gut tells me that this should be fine - the 125Vac rating indicates that it shouldn't have an issue with the contact arcing over when it's open and the 0.5A@28Vdc tells me that it has some ability to break DC current (well above my 4ma). Also, I kind of suspect that this probably isn't uncommon to do, just judging by the fact that I've seen things like thermostats, door alarms, sump level floats, etc wired to 125Vdc utility substation SCADA I/O in the past and I'm thinking it's hard to find items like those with 125Vdc contact ratings.

Obviously, a way to avoid the whole issue is to wire this alarm to a 120Vac coil interposing relay with 125Vdc contacts to pick up the digital input, at the expense of an added relay and some additional complexity. But worried that'd be making a mountain out of a molehill.

Curious to hear any advice.

 

[davidbeach]

If it could interrupt 125V DC it would have that rating. The gap may not be large enough, or get large enough fast enough, to interrupt DC at that voltage but still be fine with AC. AC, after all, gives 120 zero crossings per second to try to cause an interruption and DC never does.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[wayne440]

To me "H2 detector" and "does not have a 125Vdc rating" together make decent odds of having a very noticeable audible signal if wired directly. I suggest using an interposing relay located well below any areas where H2 might build up.

 

[bacon4life]

In other areas I have found that manufactures sometimes publish ratings that align with requirements of specific standards, rather than publishing the actual test result values. For example, one of our voltage regulators is nameplated at 11 kA, which exactly matched the requirements of the IEEE standard. After further inquiry with the vendor, the vendor provided the actual design calculations of 12.4 kA.

The right engineer at manufacturer might have more guidance for you. It is possible they tested the contacts at 125 VDC and had failures. It is also possible they never bothered testing at 125 VDC, but can provide data about the contacts directly from the component vendor.

As Wayne indicated, an explosive atmosphere has a very disastrous failure mode. It might be the typical thermostat or door alarm just fails silently, and then gets replaced without anyone ever really understanding that the contacts were not appropriately rated.

 

[cranky108]

I believe they make 2506 devices that can use 24 or 48 VDC, if that would help.

 

[thermionic1]

A SEL 2411 can be configured with different I/O cards having different wetting voltages (24-250VDC), so you could mix and match voltages in different I/O.

 

[RedSnake]

Well what you need to look for is something like this for the contact a "Rated operational power of contacts".

It is the load in W that determines how many times the contact can open together with how frekvent it has to, before it burns together.
So if the contact can brake 28VDC x 0,5A = 14W it will be able to brake 125VDC x 0.004A = 0,5 W too.
Since the gap and speed is made for a higher load 0.5A@28Vdc I think it will work.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein