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Dry contacts are "normal" metal contacts such as in a light switch or push button switch. By appropriate wiring a dry contact can usually be interfaced through controller input circuits designed for industrial sensors (NPN or PNP) and through optically isolated inputs. The voltage and current required by the circuitry to detect the state of the contact are provided by the controller.
Danger, low voltage!
There is however one very important warning about dry contacts. It relates to something called wetting current and is a significant cause of long term (and mysterious) equipment failures.
Metal contacts, when exposed to air, have a nasty habit of getting coated in oxides and tarnish. These coatings are electrically insulating. This insulation may prevent current flow, and the system may fail, with possibly disastrous results.
The good news is that some contact materials are less prone to this than others. Also, if there is enough voltage in the circuit, and the circuit can supply enough current, the contacts can clean themselves. What happens if there is enough voltage and current is that the voltage breaks down the very thin insulating layer and the (relatively high) current then blasts away the oxide. That process is called contact wetting. (I don't know why). In the telephone industry it is referred to as sealing, and generally affects crimped cable splices.
Normally the digital input circuits on electronic controllers will apply low voltages (1 to 12V) and low to very low (milliamps or microamps) currents to the external device. If you are using metallic contacts you should always check the low current ratings of the contact against the controller specification. If you can't get hold of enough information go for gold plated contacts, reed switches or bifurcated contacts. Do not select input contacts with high current ratings on the theory that more is better.
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