wiring and relays question....

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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 controller. 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). catalog #45-679
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snipped-for-privacy@aol.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.
hex -30-
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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 interchangably.
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 virtually non-existant.
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Robert Bonomi wrote:

Just a slight change in emphasis: for switching 120V loads, you must use only a single pole (don't every switch the neutral). Same for 240: double pole only.
--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com )
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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 risk.
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.
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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 delta 340.36). 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 O/C-O/L devices.
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Robert Bonomi wrote:

That's why I said "USE only a single pole." Okay, that's not really why I said it -- I was thinking 'use a single pole relay'. But it does fit. <g>
--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com )
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