SLC500 Relay Output Module

ceilingwalker

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Join Date
Mar 2010
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Phoenix, AZ
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Good day all. I have a SLC500 with a 5/04 processor and a relay output module 1746-OW16. I have an output that energizes a mag relay that applies heat to an acid dip tank.The heater in this tank has a one-shot fuse to protect it from overtemp. So the circuit goes from output module, to one-shot fuse at the tank, back to the control cabinet, then to the coil on the mag relay. What is happening is, the heater gets to temp set point, the output module denergizes, however, the coil remains pulled-in.So the tank keeps overheating and the only way to denergize the coil is to remove the wire from the coil itself or the wire between the coil and the one-shot fuse. I don't believe the module is the fault but I'm not sure. When I remove the wire from the contact point on the module the coil does not drop-out and the module does not read a voltage across it. This is not a new system, it has been in place and functioning for quite some time. I'm not sure if I have entirely ruled out the module or not, however, it seems that removing the wire and not reading voltage across that terminal point on the module,tells me the module is not the fault here. Any feedback would be greatly appreciated. Thank you much.
 
When I remove the wire from the contact point on the module the coil does not drop-out

That does pretty much absolve the module. But be sure to use protection for the module's contacts anyway.

If the module is switching DC current is it switching the high side or low side. If the low side could the one-shot fuse be shorting to ground?
 
The coil on the mag relay has a snubber across its contacts to help guard the output relays contacts.
It is switching AC current and I was just checking the one-shot, holding in my hand with latex gloves, arm resting on the 3 phase pump motor and the electricity bit me so I'm changing one-shots right now.
 
The coil on the mag relay has a snubber across its contacts to help guard the output relays contacts.
It is switching AC current and I was just checking the one-shot, holding in my hand with latex gloves, arm resting on the 3 phase pump motor and the electricity bit me so I'm changing one-shots right now.

Is the snubber across the coil, or is it across the output contacts? Are you sure the relay coil is staying in or is it sitcky contacts or snubber failure.

Snubbers across the contacts are not generally regarded as safe. They can fail as a short.
 

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