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Ballen22

Force Guided Relay

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Hello Everyone, I need some help understanding force guided relays. I understand that force guided relays are used in a lot of safety applications. I also understand that the EDM is used with a test pulse to check if the relay is working properly. What I dont understand is when the EDM is on. Is the EDM on when the relay is set (allowing current to flow) or not set? Thanks

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A force guided relay has a guide bar that mechanically links all the contacts so that they operate together. In addition, the guide bar is close to the contact pads, so if one of the contacts welds shut, all the contacts stay in that position as well, regardless of the state of the coil. Safety relays use redundant sets of force-guided relays. External Device Monitoring (EDM) is a way for a main safety relay to monitor extension relays or safety contactors (external devices) for contact weld, and it is active during the OFF to ON transition of the safety circuit. Note that all external devices must use force-guided relays. You wire the reset circuit of the main safety relay through the normally closed (NC) contacts of all external devices controlled by the safety circuit. If one of the normally open (NO) contacts of the external device weld shut, this forces the NC to stay open, so the reset circuit cannot complete and re-enable the safety circuit. An obvious question is why this is considered safe, since the contact weld has already happen keeping the NO contact closed and allowing power to flow. This is where the redundant relays come into play. One of the contacts might weld, but the chances of both redundant relays welding are negligible, so the circuit is still safe. Once the safety circuit is disabled, EDM keeps it from re-enabling. EDIT: Forgot to address the test pulses. In my experience, EDM doesn't use test pulses. Safety inputs (E-Stops in particular) use them a lot, and they're used to confirm that you don't have a short-circuit or crossed wires. Edited by JRoss

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Thank you, your post really cleared a few things up. Thanks again Edited by Ballen22

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