Switch "VT" Rating ?

[MrAl]

Hello there,


Recently i was given the following spec's for a particular switch (close ups shown in macro photo attachments):

13 amps 125vac
10 amps 250vac
1/2 HP, 125vac to 250vac
5 amps 14 VT

Note the above "14 VT".

Clearly the other ratings are for 125vac and 250vac, but i cant remember what that "VT" stands for as it's been so long since i've encountered this kind of rating. I think we even talked about this once in this forum or another forum but dont remember the conclusions we came up with.

So anyone remember what the VT stands for?

If not, feel free to throw out some ideas, guesses, etc.

 

[ronv]

I learn something new every day here.

“T” Rating denotes ability of an AC/DC switch to
handle the initial high in-rush characteristics
of a tungsten filament lamp on either AC or
DC voltage.

 

[MrAl]

I learn something new every day here.

“T” Rating denotes ability of an AC/DC switch to
handle the initial high in-rush characteristics
of a tungsten filament lamp on either AC or
DC voltage.

Hello ronv,

Did you look at the switch itself?

Here's the exact writing on the switch except for the HP rating:

13 A. 125 VAC
10 A. 250 VAC
5 A. 14 VT

So i assume that the last line there is all one specification. So in light of your previous post, what do you think they mean by:
"5 A. 14 VT"

 

[jpanhalt]

That means the switch is rated for 5 Ampere at 14V tungsten service. Hence VT is sometimes written V(T). The V has its usual meaning. The T or (T) is added to denote type of service.

John

 

[Mr RB]

I learn something new every day here.

“T” Rating denotes ability of an AC/DC switch to
handle the initial high in-rush characteristics
of a tungsten filament lamp on either AC or
DC voltage.

Nice info, thank you!

 

[KeepItSimpleStupid]

I totally forgot about that rating. I know that temperature SCR units could be purchased for tungsten loads. The cold resistance is like 10 X lower than the hot resistance of the lamp.

 

[MrAl]

That means the switch is rated for 5 Ampere at 14V tungsten service. Hence VT is sometimes written V(T). The V has its usual meaning. The T or (T) is added to denote type of service.

John

Ok that makes sense, thanks. Thanks ronv also
I rarely see this type of rating but i do remember it coming up in the past.

Also, I was hoping i could get at least low current low voltage operation for DC current as well. I guess i'll have to test.

 

[Mr RB]

Well if it is VT rated at 14v 5A that would be a switch break condition (when the bulb is drawing 5 DC amps).

So i think you could safely assume the switch would be fine for switching 12v at 5A, or anything less than 5A?

 

[ericgibbs]

hi Roman,
My understanding of the VT rating is it is the 'cold' tungsten filament lamp 'inrush' current, which can be 5 to 10 times the lamps 'hot' operating current.

There would be no point in specifying the tungsten lamp 'hot' switch off 'break' current as its the same as any other non inductive load current switch off.

 

[Mr RB]

Agreed, but my point was that the spec shows that it CAN safely break 5A DC. We don't know the turnon (make) current, but it is definitely higher than 5A.

So we can be pretty sure since it can make more than 5A and break 5A, that the switch should be safe switching other 12v loads at <=5A. (Remember the spec data in post #3 does not give DC switching specs so we have to guesstimate from the VT rating.)

 

[KeepItSimpleStupid]

Ignore any DC operation of the switch, period for any number of reasons.

This may be the primary one: the ability to wipe the contact surfaces clean.

Now let's say that it's used to switch 10 A DC at 12 V. Suppose at this level, the contacts weld because they are not made fast enough.

 

[MrAl]

Hello again,


In reference to posts 8, 9, and 10 by Eric and Roman here's my take after reading other posts here too...

The rating:
"5A. 14VT"

means the switch will operate just fine with a bulb that has a tungsten filament that draws 5 amps or less and at a voltage 14 volts or less, and that's all we have to know. By trying to specify whether or not that 5 amps is the "break current" or the "normal operating current" is futile because they are both the same, which would be 5 amps (max). So specifying either should be ok, knowing that the other is the same. I took it to mean the "normal operating current" but Eric's interpretation as the "break current" is equally valid.

We dont really have to think about the surge current, knowing it will be around 10 times higher for a tungsten bulb but will last only a short time.

So to put this into an application, with a switch rated 5 amps 14 VT or as pointed out by jpanhalt 5 A 14 V(T), that means we can go out and buy a 14 volt tungsten filament bulb that draws 5 amps nominally and expect reasonable life from the switch. That's as simple as it gets

There's still a little question however as to whether or not this works with DC as well as AC. But as one poster pointed out this rating of 14VT means either AC or DC. I do have to still wonder how well this would work with an inductive load like a motor however, where we break the current and there could be a voltage spike of very high value. Im thinking maybe some kind of snubber to keep the voltage a bit lower to protect the contacts. I think all switches have to 'wipe' at least a little though right?

I am glad i brought this up here. Looking on the web i dont see anything about this kind of rating. I see it in data sheets in different places, but not any clear description except of course for right here

AFTER reading this thread however, looking up "Switch Tungsten Ratings" turns up some very useful and interesting information:
To re-rate the switch of 125vac 1 amp for 12vdc for example i find the new ratings would be:
Resistive load: 1.25 amps
Inductive load: 1 amps
Lamp load: 0.31 amps
Motor load: 0.41 amps
Capacitive load: 0.31 amps

Where they got these numbers from i dont know so it is hard to judge the accuracy. Using this information for the original switch quoted in this thread, the rating is 125vac at 13 amps, so at 12vdc it should handle a motor load of:
13*0.41=5.33 amps.