Ron Parker wrote:

On Thu, 10 Feb 2005 14:22:30 -0700, nJb wrote:
Ron Parker wrote:

On Thu, 10 Feb 2005 12:11:20 -0700, nJb wrote:
An output can control more than one relay but I'm not sure why you would
want to have more than one.

If you're controlling a US 220-volt circuit, you really should have
a relay in each hot leg.


Why?

So that when the power is off, the power actually is off. Say, for the
sake of argument, that you have a microswitch attached to the lid, in series
with the controller output, to turn off the power when you open the kiln.

Say you're raking with a long metal tool.

Now say you only turned off one of the legs of the circuit. Guess what,
that element still has a potential of 110 volts relative to neutral (which
in most places is tied to earth) and is capable of putting up to 30 amps
through the shortest path between the tool in your hand and your foot on
the ground (or, if you have a wood floor, your hand on the shell of the
kiln, which is quite possibly also tied to neutral.)


I solve that problem by using a 2 pole definite purpose contactor with
all safety and overtemp devices wired in the coil circuit. It cuts both
legs before the control relay. On my small kiln I turn the manual
switches off before reaching in.

Jack