Below are some of the most common industry terms and their definitions. Click on a term to see its definition.


A transmission system that uses 2 wires and usually transmits positive-going pulses of some duration with a rest (or zero voltage level) between pulses. This type of a system lacks immunity to induced noise.


A transmission system that employs 3 wires and has a signal on the common to one always, but never simultaneously to both. This type of system is much less prone to noise than a 2-wire system because a signal is always present.

Dry Contacts

A relay's contacts that have no electricity applied to them from the device in which they are incorporated. A voltage is supplied by an external device or system.

Form A Contacts

A contact that is in the normally-open or non-madeup position.

Form B Contacts

A contact that is in the normally-closed or madeup position.

Form C Contacts

A set of contacts consisting of one Form "A" and one Form "B" with a single "common" ("K") contact. A Form "C" set of contacts will break one set of contacts before making up the second set.

Form D Contacts

The same a form "C" set of contacts but the contacts makeup before breaking up.

K Lead

The center or common lead of a Form "C" or double-pole single-throw switch.

Latching Relay

A relay that will stay in the last position.

Mercury Wetted Contacts

Contacts that have a very small amount of mercury around the contact point to prevent arcing or "bounce" when the contacts close. Most mercury-wetted relays (switches) must be mounted in the vertical position.


One one-thousandth of a second.

Power Source Voltage

The voltage provided to power the operating equipment. Usually 120 VAC or 277 VAC, but sometimes station battery voltages of 24, 48 or 125 VDC.

Reed Switch

A switch that is enclosed in a small glass tube, that is controlled by a magnet or magnetic field acting on it.

Sense Voltage

Also called "Wetting Voltage". The voltage used to detect the operation of either contact opening or closing as with watt-hour meters and energy controllers. Industry convention requires that the receiving device supply the sense voltage to the sending device (ie. relay to meter, energy control system to relay).

T1 and T2 Leads

The time leads coming off of most meters. Normally a Form "A" contact closure to a two wire transmission system.

Y and Z Leads

The normally-open and normally-closed contacts of a Form "C" or single-pole double-throw switch.