Need help with this circuit.

Discussion in 'Electrical Forum discussion & Blog' started by AWeitzmann, Apr 5, 2014.

1. bluebinkyMember

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That would be my first gut reaction too. However, in some situations (like conveyor belts in a factory), a lockout breaker might be required. Besides, working on any circuit after just turning off a wall switch is considered bad form IMHO.

JW are you there?

2. hjModerator & Master PlumberStaff Member

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First, you need a hot and neutral between the switches in order to power switch "B"
Second, you need a "switched" wire from switch "B" to light "B", and
Finally, you need a switched wire from switch "A" to light "A".

That is FOUR conductors between the two switches. The altermative is to make light "B" subordinate to light "A" so that it could only be turned on and off while "A" was lit, and would turn off when "A" was turned off. It is immaterial what the voltage is or whether one of the conductors is a neutral, you still need two wires to carry the "live load" and two conductors to carry the "switched" loads.

Last edited: Apr 11, 2014
3. jwelectricElectrical Contractor/Instructor

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Once the instructor has let you in on the method to be used let us know

4. bluebinkyMember

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With 208/248V, you could make it work without a neutral. My understanding is that if the neutral could be added later (conduit) then it doesn't need to be present. One question is about safety when using single pole switches in such an arrangement.

5. DonLJack of all trades Master of one

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Are you talking about 3 phase ?

Most of the light switches bought at a BB stores in the US are not rated for use on 208.

In the UK that may be a different story.

You can switch any voltage safely, If you use the correct switch.

6. hjModerator & Master PlumberStaff Member

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quote; Most of the light switches bought at a BB stores in the US are not rated for use on 208.

That statement is only half correct. "Most switches sold at a BB store" are single pole, so IF you only switch one leg, then it is 120v. But for safety purposes 208/240, etc., SHOULD be switched with a double pole switch, and the selection of those are very limited at a BB store.
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7. bluebinkyMember

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The first switch I grabbed out of my limited stash (from a BB store) says "15A-120V-277VAC". A switch on one leg of a 240V circuit with no neutral would see 240V when open.

Safety and common sense aside, it seems the OP could solve the problem without a neutral and possibly be within the code...

8. hjModerator & Master PlumberStaff Member

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quote; A switch on one leg of a 240V circuit with no neutral would see 240V when open

A 240v circuit, but definition, does not have a neutral, BUT it still needs TWO wires, regardless of what you call them. IF one of the wires is NOT a neutral, then it is the second leg of a high voltage circuit, and unless you can figure out how to have two circuits sharing the same wire in two directions at the same time, it will not work with just 3 wires. Now, telephones and model railroads used to mix DC for operating, and AC for bells and whistles, but that is probably not what you are looking for.

9. bluebinkyMember

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HJ, we both know what I mean...

10. hjModerator & Master PlumberStaff Member

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I know what you mean but it is still unsafe to switch only one leg, even assuming 240v bulbs. But ANY switch will "see FULL voltage" across the switch when it is open. That is one way to test continuity of a circuit. The only "safe" high voltage to switch with an SPST switch is 277v because it DOES only use a single hot wire and a ground. But it is not a voltage I would suggest a DIYer work with even if he did have access to it.

Last edited: Apr 12, 2014
11. DonLJack of all trades Master of one

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You could use 120V bulbs if it is fed from a 3 phase system.

Where does one get 240V bulbs ? That fits a 120V socket ?

I want some.

Last edited: Apr 13, 2014
12. Reach4Well-Known Member

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If you were telling the truth, you could screw E27 base European bulbs into your E26 socket. We all know that is not the case.

http://en.wikipedia.org/wiki/Edison_screw

13. bluebinkyMember

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Thanks, HJ. I would never suggest that anyone switch just one leg of a 240V circuit in real life...

Does the NEC forbid it in every case? Specifically for "fixed" loads like motors, resistance heaters, or whatever. (the OP did not specifically say lighting).

The instructor probably believes it is OK to switch the neutral.

14. DonLJack of all trades Master of one

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I am a big user of Lightbulbs for testing. Don't tell JW about my bad practice tho., He may delete me. lol

Most of the higher voltage bulbs come in high wattage, Or they would not need the extra voltage.

I always have used lightbulbs when working on three phase systems. I use them for single phase and DC also.

The great thing about using lightbulbs as a indicator on a 3 phase system is that you can spot a phase or ground fault problem, Before you need to get a voltmeter out to verify. A nice system will have Volt Panel meters. Most also have Lamps, for looking at a glance.

Many sockets may handle the voltage, But the heat and wattage is what really maters.

I don't really think the teacher is using 240V bulbs, But rather a trick question, With a correct answer, By his book.

Last edited: Apr 14, 2014
15. Reach4Well-Known Member

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If you were sincere this time, you could search for E27 bulbs.

I have seen nobody describe a solution that would work, other than my diode method described in post 11. Did I miss the part where 240 volt bulbs, or 3 phase power or switched neutrals would provide a way to make switch A control switch B and switch B control switch A.

I think OP is gone.

16. hjModerator & Master PlumberStaff Member

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quote; Most of the higher voltage bulbs come in high wattage, Or they would not need the extra voltage.

In most cases, the power available, such a 277v, determines the type of bulb, REGARDLESS of the wattage. How do you use a 120v light bulb in a 3 phase system, other than test phase to ground? I use a voltmeter and test phase to phase.