For each kiln to be controlled by the GB5 a power relay (usually called a "contactor") is needed. An electrician can easily install this.
There are two types of contactors:
- Mercury displacement
A Mercury displacement unit is quiet, long lasting, and more expensive. An electro-mechanical unit is quite noisy when it trips (sounds like a shot!), and its switch contacts are prone to arcing and pitting. However, it is inexpensive.
Because of the GB5's precise temperature control, the contactor relay trips frequently to achieve the desired ramping rate. Consequently, Digitry recommends Mercury relays both to reduce the noise and increase the service life of the contactor. A Mercury displacement contactor could easily outlast your kiln - or even several kilns.
How to select a contactor:
There are four decisions you must make before purchasing the appropriate electro-mechanical or Mercury displacement contactors for use with your GB5 Programmable Temperature Controller.
- Pick the type of contactor. Check how much current (amps) your kiln draws. If it draws more than 30 amps, you should use a Mercury displacement contactor. Likewise if you will normally be near the contactor while the unit is running you will want a Mercury displacement contactor. If initial cost is your major concern, you would select mechanical contactors for kilns drawing less than 30 amps.
- The next step is to choose a one-, two-, or three-pole contactor. You can use either a one- or a two-pole contactor if your kiln operates on standard house current (110 volts). You need a two-pole contactor if your kiln uses a standard 220 volt circuit. If your kiln uses a three-phase 220 volt circuit, you may be able to use a two-pole contactor, but a three-pole contactor is better; consult your electrician.
- Select a contactor that controls at least as much current as your kiln uses.
- Decide on the "coil" voltage of the contactor. This is the voltage used by the GB5 to control the contactor, which, in turn, controls your kiln. The choices usually are a 24 VAC coil or a 110 VAC coil. The 110 VAC coil is cheaper but not so safe as the 24 VAC. If your contactor-kiln-GB4 installation is physically spread out over some distance you should either select the 24 VAC coil or enclose the 110 VAC lines. If you select the 24 VAC coil, you will need an inexpensive "step-down" transformer to change the normal house voltage (110 volts) to 24 volts.