I said they can't be the "disconnecting means" but you can use them for the
controls. In his case if the blower didn't stop when the gate was closed it
wouldn't be life threatening. Personally I would use a 30a 2 pole snap switch
in a regular device box (Home Depot) as my disconnect and an SSR the
You are not supposed to use the plug as a disconnect if the motor is over 1/8
hp but you don't have to look far in you shop to see an exception to that rule.
You really only need one SSR if you have a "disconnect".
hosfelt.com has a 50a SSR for $25 that switches with 3-32vdc at a few milliamps
(any small DC wall wart would do).
email@example.comGreg (Gfretwell) wrote in message
I never said you didn't indicate that SSR's must also be used in
connection with a "disconnecting means" so don't get hostile. The
reason I posted at all was to point out that SSR's have a problematic
failure mode -- which is not a big deal for switching a DC.
Regardless of the presence of a disconnecting means, there are still
some applications where SSR's are never allowed --- even for controls.
As an example, some areas in national research labs dis-allow SSR's
as control elements --- there are areas where there is potential
radiation danger if the controls don't work.
There are three 'significant' components in the description of a relay
(and several more 'minor' ones -- that are important only if you're trying
to match an _existing_ use).
1) the power requirements for the 'coil' -- what it takes to make the
relay pull in. This determines/is-determined-by the power on the
'control' circuit. Some are designed to run on AC, some on DC, and
voltages are 'anything you might want' -- from a few volts all the
way through 440V (or higher, if you get into the 'serious' industrial
catalogues <grin>). Then there's the matter of how much _current_ it
takes to energize the coil sufficiently for the relay to 'pull in'.
typical 'low voltage' (i.e. 48V or less), moderate-power-handling relays
need from a few tens of milliamps, to a few hundred milliamps. Devices
designed for switching high power levels will draw significantly more
power (have to move heavier contacts, and over larger distances, _faster_,
all of which requires more power. the 'larger distances, faster' is to
minimize the amount of 'arcing' that goes on.) For low-voltage relays,
sometimes the current is not expressly given, instead the resistance
(in 'ohms') of the coil is specified. OHM's law, describing the relation
of current, voltage, and resistance, in a circuit lets one use these
2) the power rating for the contacts. There'll be both a voltage, and a
current rating. As long as they're both higher than what you're
actually switching, everything is fine.
3) the 'configuration' of the contacts. Here you get to deal with
'poles' and 'throws'.
'Poles' is simply how many separate wires you can switch at one time.
For switching 120V loads, you only need a single pole (there's only one
'hot' wire), for 240V single-phase, you need a 'double pole' unit (two
'hot' wires), and for 3-phase, you need a 'triple pole' unit. to
accomplish a 'latching' action -- i.e., one push-button for 'on', and a
separate one for 'off', you'll need one _more_ pole than listed above.
'Throws' is the number of positions in which there is "a path" through
the device. Without getting into 'exotic' devices, there are just the
'single throw', and 'double throw' types to consider. Single-throw
makes contact in _one_ position only. Either 'normally closed', where
energizing the relay *opens* the circuit, or 'normally open', where
energizing the relay _closes_ the circuit. "double throw" devices have
_both_ kinds of contacts. When the relay is not energized, there is
continuity from the 'common' contact(s) to the 'normally closed' one(s).
when the relay _is_ energized, there is continuity between the 'common'
contact(s), and the 'normally open' one(s).
Just to complictate things, the 'double throw' types come in two
varieties -- 'make before break', and 'break before make'. This
describes what happens _as_ the relay changes state. In the first
variety, there is a 'momentary' period when _both_ sets of contacts
(the normally open _and_ normally closed ones) are connected to the
common contact. In the second type, there is a momentary period when
_neither_ set of contacts is connected to the common.
Assuming you're going to use a switch that is 'on' whenever a blast-gate is
open, you'll need a "double-pole, single-throw (normally open), 240V 20A
contacts" relay, with a coil voltage to match whatever cheapie transformer
you can scrounge. The commonest relays will have 6 or 12 volt coils. You
can _usually_ use an AC-rated coil with DC power, although it is a GOOD IDEA
to match the specifications. Trying to use a DC-rated coil with AC power will
'almost always' *not* work right -- the typical situation is that the relay
pulls in/out 120 times every second. Quite a buzz, lots of arcing, and *lousy*
power on the 'out' side of the relay. And the 'lifetime' of the device is
Clarification: A relay with 'more poles than required' is *entirely* suitable
for a task, all else being equal. No harm done by letting a set of contacts
go 'unused'. Pete is correct about the need for care in wiring -- DO NOT EVER
switch the neutral.
The best advice, concerning electrical work:
"If you don't *know* what you're doing, _DON'T_DO_IT_! Get a pro, instead."
The work is -not- 'difficult', but 'one little mistake' can have _catastrophic_
results -- somebody gets killed, the house burns down, etc. It isn't worth the
If you _are_ considering "doing something" anyway, be sure to check local
_legal_ requirements, *and* potential effect on any insurance coverage.
I've known places where using an extension cord -- *any* extension cord,
including 'outlet strips' -- was illegal, by city ordinance. And homeowners
insurers who had a fixed policy, in the event of fires attributable to a
'defective' piece of equipment (household appliance, or whatever), of going
after the manufacturer to recover their payout.
You better unplug your PC and never use it again. If it has a hardware switch
on the PS it will usually switch both wires. That is because these are usually
120/240 capable. The code only says that if you do have a switch or breaker on
the neutral that it has to switch all conductors on the same handle and that no
pole can operate independently 240.22 & 404.2(B). It is a violation to put a
fuse in the neutral (except as motor overload protection on 3p corner grounded
These provisions are in the code to allow 120/240v equipment like PCs or
voltage tappable motor tools while still maintaining the same switches and
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