Definitions
2-WIRE
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. 3-WIRE
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. MILLISECOND
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 & T2 LEADS
The time leads coming off of most meters. Normally a Form "A" contact closure to a two wire transmission system. Y & Z LEADS
The normally-open and normally-closed contacts of a Form "C" or single-pole double-throw switch.
