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Old October 9th, 2018, 11:42 AM   #1
lesmar96
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allowed voltage on pushbutton

So what is the legal voltage allowed to a push button on the front of a control panel? We have had these discussions inshop and definitely try to use 24 or 120 when at all possible, but is there a code that actually says what is allowed?

The pushbutton contacts we use are rated at 600V. I have heard it makes a difference on whether the circuit is contained or leaves the enclosure.

can someone help us?
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Old October 9th, 2018, 12:00 PM   #2
Nova5
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Rated doesn't mean you should actually use it at that level. Hell 24Ga wiring carrys a 600V rating. are you really gonna push that through it?
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Old October 9th, 2018, 12:35 PM   #3
lesmar96
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I was just indicating that using the switch at 240V would not be beyond its rating. I am mostly concerned about the safety implications of having that high of a voltage on a device that someone touches.
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Old October 9th, 2018, 01:53 PM   #4
kalabdel
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I don't know about code or regulation but I look at these questions from the point of moral and legal responsibility and in this case my judgment 24V is safe, 120V is border-line and anything else is asking for trouble. I don't know if this is just my experience or if it is wide spread but I noticed that North American equipment have 24V on most controls and sensors and European can have either 24V or 120V and I got zapped a couple of times because of my sloppiness and expectation of 24V.

Last edited by kalabdel; October 9th, 2018 at 01:55 PM.
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Old October 9th, 2018, 01:59 PM   #5
Greg7683
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I would guess if they sell them then it would have to be alright to use. I would just us a contractor with a 24V coil that's what we do if we need to use a button for one of our 240V motors
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Old October 9th, 2018, 02:05 PM   #6
Saffa
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Here, where our phase to neutral voltage is 230V, that is certainly quite common to have through a pushbutton or selector switch on a cabinet door. Yes, a lot of control stuff is now 24V and that's what we use for new stuff but I see absolutely no issue with 230V to earth at a terminal block.

Of course a metal panel door must be solidly earthed.

Electricians that make assumptions without testing aren't going to have long careers in my humble opinion. You should not be touching anything until you've proven it to be safe and isolated it in such a manner that it cannot become live again.
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Old October 9th, 2018, 02:17 PM   #7
geniusintraining
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This may help...

Quote:
Power coding of the switching capacity of control circuit devices, such as auxiliary switches,
position switches, pushbuttons, auxiliary contactors, time relays, overcurrent relay contacts,
time counters, insta contactors, position switches, etc. are stated with the letters A to E for
AC and N to R for DC devices, plus the voltages 150 V, 300 V and 600 V; this means, for
example, the 150 V switching capacity shall be selected for devices in 24 V control circuits.
A device, marked with a switching capacity of A600, can make at 460 V 15 A and break at
1.5 A.
The make and break capacity of these devices is stated on the nameplate for different
voltage levels. It is similar to the details according to IEC e.g. AC-15.
Page 84 http://www.siemens.fi/pool/products/...0-englanti.pdf
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Old October 9th, 2018, 03:58 PM   #8
Tom Jenkins
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Quote:
Originally Posted by Nova5 View Post
Rated doesn't mean you should actually use it at that level. Hell 24Ga wiring carrys a 600V rating. are you really gonna push that through it?
A clarification is required.

Voltage rating identifies the voltage difference between a device or wire that can be withstood before an arc will jump, plus, of course, a factor of safety. Any material will conduct. Lightning occurs because the voltage difference between cloud and ground exceeds the insulating capability of the air.

Wire gauge or contact rating is established by the current carrying capacity of the component that can be handled without excess heat creating meltdown or excessive erosion or other catastrophic failures. Ohms Law applies, so putting 600 V on a 24 gauge wire carrying 1/10th of an amp is just fine since that is well below the wire rating.

Putting the rated voltage on a pushbutton or other device is just fine. After all, how do you think they manage to get disconnect levers on medium voltage switchgear?
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Old October 9th, 2018, 04:05 PM   #9
just the cowboy
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Back in the 70's many combination starters had 480 volts on the pushbuttons, with 480 volt coils on the contactors. Man I hated testing them with a wiggy because of the flash and clack.
Since then I thought you needed a control xformer to bring it down to at least 120V but not sure if it is code required or code suggested.
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Old October 9th, 2018, 04:38 PM   #10
mass89
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I've seen a number of new installations with 110v control, 24v is widely favoured now. If it was a single pushbutton to pull in a contactor for 230v then I would just use the 230v, of course ensuring correct earthing.
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Old October 9th, 2018, 04:39 PM   #11
Toine
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Quote:
Originally Posted by kalabdel View Post
European can have either 24V or 120V and I got zapped a couple of times because of my sloppiness and expectation of 24V.

Common voltage in Europe between neutral and phase is 230VAC rather than 120. Between two phases we have 400VAC. Other than that you are completely right, and I have made the same mistake once, assuming a panel had 24V control voltage while in fact it was 230V. I wasn't hurt, nobody else either. I did realize that I was lucky, learned my lesson and have been careful ever since: never assume - always test.
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Old October 9th, 2018, 04:57 PM   #12
Remrun
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As just_the_cowboy said there are are facilities at least in the US that have 480VAC pushbuttons for start/stop control of 480V starters. I would guess most of these installations were done in the 70's and before. My guess is that prior to having automation where I/O cards don't switch 480VAC very well it saved having to install control power transformers in every MCC bucket or developing some kind of lower voltage distribution and protection scheme. That being said there are certainly safety concerns and I certainly wouldn't design one that way.
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Old October 9th, 2018, 05:06 PM   #13
cardosocea
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I would hope that if you have a PLC already, the button voltage would be 24Vdc.



There shouldn't really be a reason for anything larger than that... although I see it every day, in hazardous areas, no less.
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Old October 9th, 2018, 05:09 PM   #14
OkiePC
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Quote:
Originally Posted by Remrun View Post
As just_the_cowboy said there are are facilities at least in the US that have 480VAC pushbuttons for start/stop control of 480V starters. I would guess most of these installations were done in the 70's and before. My guess is that prior to having automation where I/O cards don't switch 480VAC very well it saved having to install control power transformers in every MCC bucket or developing some kind of lower voltage distribution and protection scheme. That being said there are certainly safety concerns and I certainly wouldn't design one that way.
I worked on one a few months ago that is less than 2 years old. 480VAC on a selector switch and start button going to the coil. It was a prefabbed C/H starter panel that their local electrician purchased and installed.
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Old October 9th, 2018, 05:42 PM   #15
lesmar96
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Quote:
Originally Posted by OkiePC View Post
I worked on one a few months ago that is less than 2 years old. 480VAC on a selector switch and start button going to the coil. It was a prefabbed C/H starter panel that their local electrician purchased and installed.
I guess this is really what prompted my question and several of you have brought it up. Starter panels..........

When all you have is a starter panel mounted on the wall, with a start/stop buttons in the front, it is easiest to simply use the line voltage.

We get these requests various times and I was not sure what is the proper way of doing. It is a pain, although the safest, to use a control transformer.

What we build and sell, generally is a poly enclosure, so if that button would somehow fail, the next person to touch it could potentially be the next path to ground.
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