Maximum switching voltage

[MasBagong]

Hi,

I need to switch a load of 433V with relay contacts. The maximum switching voltage of the contact is 250V. Can I use two contact in series for this application? Will the maximum switching voltage of the contact become doubled? Any reference (IEC, ANSI, NEC, etc.) for this??

Thanks & best regards,
MasBagong

 

[eric7379]

No, you cannot use 2 contacts in series to cure this problem.
No, the maximum switching voltage will not double. Think about it, if you have 433V across the first set of contacts, you will still have 433V across the second set of contacts. The voltage will not change.

What about using a contactor?? The contacts in a contactor have a higher voltage/amperage rating for the contacts than what a relay does.


For example, looking at an ice cube relay that I have in front of me, the maximum rating for the contacts is 250 VAC/ 5 A. Looking at a Square D contactor that I have, the maximum rating for the contacts is 460/575 VAC at 10 HP.

If there is an issue of the coil amperage of the contactor being too much for whatever is energizing it (PLC, proximity sensor, etc.), you can use a slave relay. Have the PLC or prox. to energize the relay. Use the relay contacts to energize the contactor coil.

Don't have any standards or code references for you. Don't have any of the books in front of me. I'm going off of experience.

 

[MasBagong]

Hi eric7379,

Thanks for your response.
I found in a catalog for mcb, maximum voltage for single pole is 60V and for 2-pole is 125V. It should be the same for relay contact, shouldn't it?

Rgds,
MasBagong

 

[dbuckley]

No, it never is, because the two switch poles in series would not open at precisely the same time, so one pole opens first, and it breaks 433V. When the second pole opens (a fraction of a second later) it has 0V across it.

 

[winnie]

I am in _slight_ disagreement with those saying that it isn't possible to connect two poles of a control device in series to get a higher voltage rating.

First to emphasize the bulk of my agreement: MasB do _NOT_ do this in any sort of installation.

Now my little bit of disagreement: sometimes, with some switching devices, this is a possible way to use a device. It all depends upon the manufacturer's design and testing of the switching system. If the device has been designed and tested to be suitable for such a connection, then you can do this. I have only seen this for three phase AC breakers which were also rated for DC use. I have also seen it in circuit designs for high voltage transistor switching systems.

In any case, it comes down to the same thing: you must find a device that is rated for the voltages that you are using. It _may_ be the case that a given device will have a higher voltage rating if poles are placed in series, however generally this is not true.

-Jon

 

[skeshesh]

While Jon's comments are stated so universally its hard to _completely_ disagree with I think it's a bad idea all around. I do agree that in a case where system characteristics, manufac. recommendation AND purpose of contacts (how fast they have to act, how often, what do they break, why, etc.) have been studied at lenght you can impelement something like this safely. However, it requires an experienced engineer and a bunch of time/effort.

 

[LarryFine]

One of my engineering friends does tout placing contacts in series, but for extra current capacity and contact life, not for increased voltage capacity.

 

[Jraef]

You can get higher switching voltages for DC by putting contacts in series. This is because one of the big challenges in switching DC is the arc length, because unlike AC that crosses zero 120 times per second, DC never does, so ONLY the air dielectric from contact separation extinguishes the arc. Putting contacts in series increases the total dielectric capability of the switch. So for example (one that is coming up a lot lately) if you are looking at DC "Solar rated" disconnect switches, you can use a 3 pole switch to switch 3 separate 250VDC circuits. But if you need a 600VDC rating, you have to run one circuit through at least 2 of the poles in series.

None of that applies to AC however. Usually, the issue on AC switch ratings is not just contact separation, but also component creepage distance. At 600V, you must have at least 1" of total surface area from one live current carrying component to the next. That's why, on 600V rated contactors, you see longer barriers between poles. A relay rated 250V will not have that, and in particular, standard "ice cube" relay bases do not have that kind of separation between the screws either. So what can happen if you apply 480V to a relay rated for 240V, is that you may get a flash over.

 

[skeshesh]

You can get higher switching voltages for DC by putting contacts in series. This is because one of the big challenges in switching DC is the arc length, because unlike AC that crosses zero 120 times per second, DC never does, so ONLY the air dielectric from contact separation extinguishes the arc. Putting contacts in series increases the total dielectric capability of the switch. So for example (one that is coming up a lot lately) if you are looking at DC "Solar rated" disconnect switches, you can use a 3 pole switch to switch 3 separate 250VDC circuits. But if you need a 600VDC rating, you have to run one circuit through at least 2 of the poles in series.

None of that applies to AC however. Usually, the issue on AC switch ratings is not just contact separation, but also component creepage distance. At 600V, you must have at least 1" of total surface area from one live current carrying component to the next. That's why, on 600V rated contactors, you see longer barriers between poles. A relay rated 250V will not have that, and in particular, standard "ice cube" relay bases do not have that kind of separation between the screws either. So what can happen if you apply 480V to a relay rated for 240V, is that you may get a flash over.

Great point, didn't quite think about that.

 

[LarryFine]

But if you need a 600VDC rating, you have to run one circuit through at least 2 of the poles in series.

I've wired fork-lift battery chargers that could be wired for 1ph or 3ph operation, and on 1ph, one line went through the 3p input breaker that way.

 

[glene77is]

would not open at precisely the same time

DBuckly,
That was my first thought.
I agree.

 

[glene77is]

One of my engineering friends does tout placing contacts in series, but for extra current capacity and contact life, not for increased voltage capacity.

Larry,

Thevin says that current is the same in a series circuit.
There should be no 'extra current capacity'.
It would appear that there is extra voltage capacity,
but you still have the problem of getting them to all 'open' at the same instant.

Contacts in parallel would have extra total current capacity,
but you still have the problem of getting them to all 'open' at the same instant.


Comments are welcome.

 

[ohmhead]

Well ac or dc 50 hz or 60 hz

Voltage between multi air gaps to contact points if in series will divide the total voltage between metal contacts points that conductive air gap between them if each gap is spaced the same between contact points .


Kinda like a rotory switch on a high voltage tesla coil the gaps divide up the total voltage as one contact alone would burn out at that high voltage but 6 or 8 gaps will conduct at a lower voltage level but series up to gives total voltage .

Current has no point in this issue only voltage the point that a voltage will jump the gap and make a connection .

Its called the old spark gap transmitter they used muti contacts to switch CW

 

[Besoeker]

Hi,

I need to switch a load of 433V with relay contacts. The maximum switching voltage of the contact is 250V. Can I use two contact in series for this application? Will the maximum switching voltage of the contact become doubled? Any reference (IEC, ANSI, NEC, etc.) for this??

Thanks & best regards,
MasBagong

I'm with the others here. Don't do it.
Use the relay to drive a contactor with a coil voltage of less than 250V.
Such a contactor isn't expensive. I can buy a 3-pole 9A AC3 (inductive) rating or 21A AC1 (resistive), 110Vac coil for around ?10.

 

[ohmhead]

Well i agree Besoeker as to the op use of a switch . Dont do it !!

My point was it does work on high voltage switching dividing up voltage now a question for you even today switching at very high speed fast switching a spark gap or a IGBT which is better in your opinion ?


Were only talking 60 hz high voltage not serial or digital signals AC only ?