Wouldn't a closed circuit have some kind of power going through it or how would it know the circuit is open or closed?
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I am assuming the NVR or Cameras would have a reference signal or supply there own voltage to a senor. I think what nayr is saying , do not induce additional voltage from and external source. A dry contact or relay is only used to make the connect between the two leads coming from the camera or NVR. When a relay is energized by external power it puts contacts closed to complete the circuit attached to it.
Example of a on off relay setup : Contact a has eternal power , contact b is negative. External power is applied and creates a magnetic field that pulls a switch closed. On the other side of the relay contact c and contact d close to complete a circuit wired on that side. No Power flows from a/b contacts to c/d contacts.
Simple right ? Wrong , relays can be designed in different ways and have multiple contact points. Relays can be wired to apply power from , lets say f contact and allow it to flow thru c or d contact. So if you set a relay up like this you are now inducing external power into your camera or NVR and will have a nice paper weight.
Even if you think you have a relay wired the correct way you should always tested the terminals / contacts you want to connect to before you damage equipment. Hence break out the Fluke or lick the terminals...
Ok gang I think I explained it in simple cave man terms.
Please correct me if I am wrong.