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I am using a safety PLC from Schneider and it is connected to a safety output relay with model number XPSMCMRO0004. The datasheet mentions that the min and max switching current is 20mA to 6Amp respectively. I wanted to ask how is switching current different from Load Current. As i understand switching current is just to switch the coil of the relay and it should be different or independent of the load current. Or are they the same in this case? Any help will be appreicated.

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In this case, switching current is the current that the contacts can safely and reliably switch. There is a low current limit (20 mA) as low currents may not reliably "burn through" oxidation or other contamination on the contacts.

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  • \$\begingroup\$ @TonyStewart.EEsince'75 THank you. So i can easily drive a 3 amp load from it without any issues.? The problem has come up with all of these that we have used in field. The output module has failed and reset everytime an output is generated through the safety PLC. We are driving a hydraulic solenoid valve directly from the PLC without buffer relay in between. is the back emf from the valve be affecting the safety outputs? \$\endgroup\$ – Ammar Surti Aug 17 '17 at 18:11
  • \$\begingroup\$ is that surge or SS? \$\endgroup\$ – Sunnyskyguy EE75 Aug 17 '17 at 18:12
  • \$\begingroup\$ @TonyStewart.EEsince'75 After checking the back emf, the valve when shuts sends upto 150 volts back to the PLC. But this being a safety output plc, shouldnt it already have protection in it? i have opened up the output relay and i see 4 relays on it, so i pretty sure that the relays are already providing the safety from the back EMF. \$\endgroup\$ – Ammar Surti Aug 17 '17 at 18:14
  • \$\begingroup\$ I doubt there is any snubber in the relay. L/R arc duration reduces MTBF. Check with OEM . consider 1uF motor start cap across valve. \$\endgroup\$ – Sunnyskyguy EE75 Aug 17 '17 at 18:36
  • \$\begingroup\$ @TonyStewart.EEsince'75 The motor side is fine, the solenoid valve output gives out the error and shuts the module off. \$\endgroup\$ – Ammar Surti Aug 17 '17 at 18:49
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Switched current affects failure modes due to oxidation below wetting current or above max elevates contact temperature during arcs.

Load current is steady state. For example a light bulb has surge up to 10x turning on and diode bridge cap loads are similar.

Switching events are usually worst case.

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