Member Discussion

Wet Vs. Dry Contacts And Their Use In Access Control

Wet contact vs. dry contact - Is Access control hardware always "dry"

  • Should I be worried about other systems that I may be tying into?
  • Is there an easy way to tell if a contact is wet or dry?

There is some ambiguity with terms like this, so some may use them differently.

In my experience, a 'wet' contact is one that passes voltage as soon as it is made, while a 'dry' contact is one where voltage isn't sent just by completing the circuit.

Think in terms of a garden hose connected to a spigot. A 'wet' contact is connecting a hose to a spigot that is flowing water. A 'dry' contact is connecting a hose to a spigot that is turned off. In electronics, electricity would be equivalent to water.

Most security/access devices I see use 'dry' contacts. This is generally safer, but also because access control uses logic to determine when something is 'on' or 'off'.

Yes ... a "wet" contact has a common leg that is "hot", or I guess to stay consistent, "wet". This means that as soon as the relay closes, the current wired to the common leg of the relay is passed through to the load. In a "dry" relay configuration, there is no live voltage wired or routed to the common leg of the relay and when the relay closes, there is nothing but a closed, "dry" circuit.

Beyond this, there are various configurations of control relays that may or may not have a visible and obvious source of power wired to the common leg of a control relay on a controller ....it differs from controller to controller since there are some that internally route voltage to the common leg .. touch the common terminal of the relay with the red lead of your volt meter and the black lead to ground terminal on the controller ...if there is voltage showing, then you have a wet relay.

IMHO I would also add that an interpretation of a wet output would also be a transistor switch, either low power to drive a solid state relay or higher current to drive a lock.

http://hyperphysics.phy-astr.gsu.edu/HBASe/electronic/transwitch.html#c2

Interesting topic: Just to clarify in common usage (because I'm not sure myself).

Take for example an Altronix power supply commonly used in access control with a FACP (Fire Alarm Interface) and fused outputs to locking devices.

The N/O and Common of a lock output to a strike would pass voltage to the lock when triggered. Is that a "wet" contact?

The FACP contact is a trigger input of sorts that will be connected to the Fire Alarm panel auxiliary output to kill power to the locks if triggered. Would that be a "dry" contact

Wet= provides power (usually Limited) from the mother board.

Dry= you are providing power or according to your circuit need open or closed when connected to the common point

I have always had a personal policy to not 'draw' from the Mother board on any system (Access or Burglar) unless I really had to. It is not a must do but it cannot hurt anything as long as the technician remembers that common is common.

FWIW, these strange but familiar terms, (wet, dry), come to us from telcom. You might purchase either a 'wet pair' or a 'dry pair' of transmission lines from the phone company. A wet pair came powered by the phone company, a dry pair needed to be powered.

In the case of the wet pair, the power itself came from a battery that was placed on the line, i.e. a 'wet cell'.

Thanks everyone. I am starting to see the value in these discussions. Great to see everyone's diverse experience.

In either case, there is a current that will flow and the type of contact must be able to handle the current load.

For example, a 'wet' IO could be defined as being able to source/sink 100ma to/from a destination. If the destination device demands MORE current than that...trouble is brewing.

A 'dry' IO may be switching a motor or other high current load. The contacts will be rated for a certain amount of power. You will need to manage the capability of the device to the application.

If you don't do this, your IO devices will fail.