AB relay output contact ratings

whumphrey

Lifetime Supporting Member
Join Date
Aug 2011
Location
Lincoln, NE
Posts
35
Hello,
We are designing a SLC to ControlLogix replacement and running into some possible issues with output modules.

Current system uses 1746-OX8 modules and switches field loads (primarily 120VAC directly. The contact ratings on the 1746-OX8 are 2A per contact.

If we are reading correctly, the 1756-OW8 and 1756-OW16 have contact ratings of 1A per contact.

So far we've not found a 1756 relay ouptut module that retains the 2A per contact rating.

It is not very practical for to verify each and every actual load current and not practical to install interposing relays in the existing cabinets.

Has anyone experienced this same or similar issue? Suggestions?
Thanks.
 
whumphrey said:
...If we are reading correctly, the 1756-OW8 and 1756-OW16 have contact ratings of 1A per contact.

The 1756-OX8I and the 1756-OW16I are the only two available relay output contact modules for the ControlLogix.

They are both rated for...

1.5A @ 120V AC 50/60 Hz
0.75A @ 240V AC 50/60 Hz

You quoted 1A, which is for 1 A @ 5…30V DC.

EDIT1: Sorry had not read your last reply...

EDIT2: A most likely reason for down rating these relay output modules contacts is to dissuade users from being tempted to drive heavy/dirty AC loads directly from the output contacts. The more they give you the more you "try".

Your obviously a bit concerned about driving the existing AC loads, even with a 0.5A drop, and are reluctantly contemplating the use of interposing relays, and will most likely be advised to do so by many here, either way.

Rockwell's job is done here I feel?

Regards,
George
 
Thanks for reply. Forgot to mention we had investigated that one too. The document you linked is from 2003. There is a more current version with specs that appear to show a 1.5A @ 120VAC rating but still short of the 2A rating of the 1746-OX8.

http://literature.rockwellautomation.com/idc/groups/literature/documents/td/1756-td002_-en-e.pdf

Rockwell tech support says the module was re-designed and now has a lower contact rating.

True?


Sorry about that. I got it from a quick Google search. Truth is, you should make some effort to identify the actual loads on these outputs and use interposing relays where necessary. You probably won't need them on most of your outputs, but I typically specify IRs on larger motor starters and solenoids.

That being said, AB really should offer an equivalent replacement module if they are going to advocate upgrading SLC systems to ControlLogix.
 
Have you looked at using a 24Vdc output card, with a AB Field Termination board that has the interposing relays built into it? You can even buy the cable between the IO card and the Field Terminatinon board pre terminated. They have quite a small footprint.
 
Thanks all. We have about 36 PLCs to upgrade each with an average of about 16 digital outputs. The biggest obstacle is panel space. The pre-wired IFM solutions are just to large for most of the installations. I think what we are looking at is a 2-tier terminal block style relay. This should be the smallest footprint.

I wonder if I can purchase a pre-wired module swing-arm that is just fanned out wiring at the other end? This would save a little wiring effort.

It is disappointing that AB does not have a better migration solution.

Thanks!
 
I don't mean to needlessly complicate this – but - besides just the current capability - you should also take a look at the amount of offstate "leakage current" involved in your replacement modules ... depending on the types of loads you're connecting, that may - or may not - be a factor in your choice ...

yes, I realize that we're talking about "relay" types of output modules here - but some types of these have "arc suppression" built into the "contact" circuit - which MIGHT contribute to "leakage current" issues ...

SUGGESTION: whatever route you decide on – order a sample and TRY IT OUT before you jump into the deep end of the pool ... if you have various types of loads – try it out on each of them ...

note that some of the very smallest interposing relays can be "held on" by the output module's leakage current – even when the processor is trying to turn the output OFF ... since you're looking for the "smallest footprint" solution this could become an issue for you ...

good luck with your project ...
 
Last edited:
Quite true Ron,

But it's not really a widespread issue with many modules to be considered.

As I said, there are only two relay contact output modules available for the 1756 range.

So in fact, the only module to be concerned with is the 1756-OW16I, as it is the only one of the two modules that have snubbers built in.

The 1756-OW16I is notorious for its leakage current holding on relatively low impedance loads. Because it has built-in snubbers on its relay contacts, It has a typical leakage current of 1.5mA in the off state.

Most starter and solenoid coils are sufficiently rated for an off state leakage current as low as 1.5mA, but certain miniature interposing relay coils, such as the slim terminal style ones, can indeed be held on.

Once you ensure their off state current rating is sufficiently high enough, the leakage current should not really be an issue.

EDIT: In retrospect Ron, I think you were referring to all output modules that have leakage currents, and not just the "contact" output modules?

If so, and our friend deviates from trying to migrate contact to contact type output modules, then yes, your advice is highly relevant and widespread.

Apologies if so.

Regards,
George
 
Last edited:
Greetings George ...

basically I just took a QUICK look at the specs for the SLC modules the OP mentioned - and then a QUICK look at the specs for at least one of the ControlLogix modules that had also been mentioned in the responses ...

since no one else had mentioned the potential for a "leakage current" problem with the new modules I thought that I'd bring that point up for discussion ...

basic idea:

some people have the MIS-conception that simply using a "contact-type" output module is guaranteed to eliminate any "leakage current" problems ... that is NOT always correct ...

I've also seen people install tiny little (itty-bitty) relays as "interposing" relays - only to have them "stick ON" when the PLC tries to turn them OFF - due to the "leakage current" effect ... (some models of these tiny little relays don't always present the necessary "off state load" necessary to make the coil reliably "drop out") ...

since the OP hasn't specifically told us exactly what types of loads are being controlled here – we're all just speculating on what would be his best course of action ...

I'm not sure that I've ever mentioned it – but I worked for an Allen-Bradley distributor for about eight years ... I can well remember some of the turmoil that occurred when customers would try to return parts that they had mismatched – and that wouldn't work correctly once they had been connected and tried out ... (ugly situation) ...

it certainly sounds like the OP understands most (if not ALL) of the concepts involved here – but then again – he DID come here asking for advice ... I'm just bringing up another piece of the puzzle which might (or might not) prove helpful ... I know (from experience in helping other people solve these types of problems) that some of the specifications get overlooked ... that can turn out to be quite expensive – and professionally embarrassing – to the "overlooker" ...

you and I are on the same page ... we're both just trying to make sure that the OP (and other readers in the future) don't skip over some of the details when taking on a project like this ...

party on ...

.

output_module_comparison.PNG
 
Ron Beaufort said:
...you and I are on the same page ...

Ok, so let's turn to the next page?...

So to add to Ron's aforementioned and pertinent potential for leakage current "Hold On", and to provide a solution when faced with this issue...

Ron Beaufort said:
...note that some of the very smallest interposing relays can be "held on" by the output module's leakage current – even when the processor is trying to turn the output OFF ... since you're looking for the "smallest footprint" solution this could become an issue for you ...

If tight for space, and you've found that perfect waif of an interposing relay, but its impedance is way to low for your leaky output signal, then you can add a "Load" or "Bleed" resistor in parallel with the coil. Using the output's current leakage spec., and good old Ohms Law, you can calculate the necessary resistor's impedance and wattage...

24498 - Calculating Load Resistor to Bleed Off Leakage Current
Access Level: Everyone

There is a very good example there of a situation where the combination of a leaky Output, wired to an Input with a low max. off state current rating, can end up with the Output holding the Input ON, when the actual Output signal is in the off state.

These are two very commonly used I/O modules as well. So it goes to show how the manufacturer does not sit down and plan out, or design out, the potential for this between their own modules.

But then again, interposing between controllers is very good practice, in my opinion.

Regards,
George
 
I would take the opportunity to add the relays while you are updating the panels. One reason not mentioned (at least as far as I saw, I might have missed it) is that if a relay does fail then you aren’t replacing the whole module but rather just the failed relay. It provides a level of flexibility as well. While it may never become an issue, with a DC output module and interposing relays you can change an outputs voltage type and rating (AC to DC or 1A to 5A) without having to add a different module or change one out (just change out a relay). If it were me I’d upgrade the panel with relays while going through the process of changing out the processors.
 
Firejo said:
...if a relay does fail then you aren’t replacing the whole module but rather just the failed relay...

Yes Firejo,

In a word, we are talking about "Isolation" here. The relay acts as a barrier between the field wiring and the sensitive PLC wiring. Let's say the relay fails from an unsuppressed load in the field. It's much easier, and cheaper to change out an interposing relay, than an Output module. If an Output module is not an Isolated module i.e. individual wiring per point, then a field induced back fault can potentially take out more than one Output, if their supply wiring is grouped.

An ideal PLC enclosure would entirely isolate the Inputs, and the Outputs from the field. This would involve also driving interposing relays from Input signals, and driving the actual PLC Inputs through the relays. This way, all PLC I/O wiring is kept in the enclosure.

I'm talking about the same voltage here - say 24VDC Input signal to 24VDC relay, switching 24VDC to Input module, just for Isolation.

Overkill?

I wonder, does anyone do it as standard?

Regards,
George
 

Similar Topics

N
I was checking the manual, and found that the Relay type module can stand up to 2Amp of load, 24VDCv and upto 220VAC. What I am confused about, I...
Replies
20
Views
8,125
I’ve got some devices that are triggered by shorting pins to common. There is 24 volts on each pin and right now I am using a relay output card...
Replies
8
Views
2,496
I have a Automation Direct Click Plus that you can buy option CPU slot modules for input/outputs. The slot modules have the options of sinking...
Replies
9
Views
2,732
Troubleshooting a Eaton Easy719-AC-RCX. Have three digital inputs and one relay output. PLC input power 120VAC. Each digital input has a 120VAC...
Replies
6
Views
1,957
Hi all, A client is asking if I can move a motion sensor from an existing alarm dialer to a PLC discrete input. Looking up the datasheet, it...
Replies
5
Views
2,288
Back
Top Bottom