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Old April 19th, 2017, 08:50 AM   #1
SoftwareJanitor
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ControlLogix: 120V AC Output putting out 87V ?

Is this the card, or what's driving the card? It's just a permissive being sent from one building to another.
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Old April 19th, 2017, 08:56 AM   #2
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A bit more info will help a lot with suggestions

PLC? Module?

But gut feeling is a loose / bad neutral (or whatever its called on your side of the pond )


Brian
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Old April 19th, 2017, 09:02 AM   #3
cwal61
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1. Is the output on or off when reading 87V?
2. Are you using a digital meter?
3. You most likely are using a solid state card instead of a relay card.

If off and using digital meter and a solid state card it's phantom voltage being read by the digital meter. They measure the potential between the leads. That's what you'll see with no load. If you had a old Simpson 180 meter (analog type) it would put enough resistance on the output to draw it down to 0V. When the output is off.
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Old April 19th, 2017, 09:12 AM   #4
GaryS
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[It's just a permissive being sent from one building to another. ]
That about says it all. Ground is not always ground. different potential between the 2 buildings.
This just shows why proper grounding (Bonding) in electrical systems is important.
Run a grounding conductor between the 2 buildings
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Old April 19th, 2017, 09:16 AM   #5
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Quote:
Originally Posted by SoftwareJanitor View Post
Is this the card, or what's driving the card? It's just a permissive being sent from one building to another.
What is the output module Catalog Number?

Is it a TRIAC type?

You have to remember that a TRIAC is a current switch; a load connected to a TRIAC in the OFF state (even if it measures 87 VAC across) is not 'energized" since the current path is open.
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Old April 19th, 2017, 09:17 AM   #6
Ron Beaufort
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I'm betting (more than pocket change - but less than the rent) that he's dealing with a TRIAC module - and "leakage current" ...
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Old April 19th, 2017, 10:11 AM   #7
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Quote:
Originally Posted by Ron Beaufort View Post
I'm betting (more than pocket change - but less than the rent) that he's dealing with a TRIAC module - and "leakage current" ...
Yep. I've seen leakage current surprise quite a few folks over the years. Triac outputs are the worst, but surge suppression circuits can present leakage current even in a relay output.
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Old April 19th, 2017, 10:13 AM   #8
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Quote:
Originally Posted by GaryS View Post
[It's just a permissive being sent from one building to another. ]
That about says it all. Ground is not always ground. different potential between the 2 buildings.
This just shows why proper grounding (Bonding) in electrical systems is important.
Run a grounding conductor between the 2 buildings
Ground continuity is crucially important, but it may also be necessary to add a resistor in parallel to the input in order to load the output circuit.

Either that, or employ an interposing relay.
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Old April 19th, 2017, 10:34 AM   #9
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From the way it was posted I take it that they were energizing a 120Vac output and only reading 87V at the receiving end in another building.
Baring voltage loss due to wire resistance ( if it's just signal there would no voltage drop )Then it's clear to me that the 2 building are at different ground potentials.
this is why all points in an electrical system must be grounded to same point.
if the 2 building have a separate service then you must isolate the 2 systems
if they get connected together there could be big problems
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Old April 19th, 2017, 10:41 AM   #10
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you gotta know when to hold 'em - and know when to fold 'em ...

well, the OP didn't tell us whether the output was OFF or ON when he got that 87V reading - so the "ground" issue might certainly be correct ...

and ...

he didn't tell us whether the "input" on the other end was regarding the 87V reading as an ON or an OFF signal ...

and ...

he didn't tell us which end of the wire the 87V reading was taken from (output module end - or input module end) ...

but ...

I've just got a "hunch" that it's leakage current - and since I'm not betting much, I'll stand ...
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Last edited by Ron Beaufort; April 19th, 2017 at 10:47 AM.
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Old April 19th, 2017, 10:59 AM   #11
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Quote:
Originally Posted by Ron Beaufort View Post
well, the OP didn't tell us whether the output was OFF or ON when he got that 87V reading - so the "ground" issue might certainly be correct ...

and ...

he didn't tell us whether the "input" on the other end was regarding the 87V reading as an ON or an OFF signal ...

and ...

he didn't tell us which end of the wire the 87V reading was taken from (output module end - or input module end) ...

but ...

I've just got a "hunch" that it's leakage current - and since I'm not betting much, I'll stand ...
I agree, but if it is a ground potential issue, then an interposing relay in the "input panel" with both the output and the neutral sourced from the "output panel" would resolve the issue.
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Old April 19th, 2017, 07:16 PM   #12
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Quote:
Originally Posted by SoftwareJanitor View Post
from one building to another.
100 yards?
quarter mile?
half mile?

24g CAT5 cable?
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Old April 20th, 2017, 12:16 PM   #13
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Call off the calvary...

As I've explained previously, I'm not a PLC guy, so I guess this was a false alarm.

It's a ControlLogix L73.

The card is a 1756-OW16I, 10-265V AC, 5-150V DC Isolated Relay.

The run between buildings is 300-400' ??

It was reading 87 volts with the signal OFF.

I had forced it ON in the PLC, but it really wasn't "ON" until I realized I had to "enable all forces" to make the "ON" turn red in the ladder. I *knew* I had the right point, and I saw the "ON" appear on the ladder output when I forced it, but didn't know it wasn't active until it turned red! Oh well!

Thanks for the quick replies, though ...
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Old April 20th, 2017, 12:52 PM   #14
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Just to confirm what others have said. Your module does have a leakage spec, see pdf.
Attached Files
File Type: pdf leakagespec.pdf (119.6 KB, 25 views)
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Old April 20th, 2017, 01:40 PM   #15
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Let me follow up with this
the output is an OW16 card that's a relay output ( NO leakage)
if you were reading 87V at the receiving end with the output ON forced or otherwise then you have a serous grounding problem that needs to be addressed.
if you are reading 87V with the output OFF then you have some other serious problems with your system the needs to be addressed ASP. because you have a floating voltage someplace this floating voltage can kill you.
as for leakage, all solid state devices leak when off and the have a voltage drop across them when turned ON the current through them and the voltage drop when ON causes the heat in the device ( Heat is a bad thing here )
That's why they are called semiconductors ( Never fully ON and Never fully OFF)
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