Using a Switch at Different Voltages

I realize that the arcing when breaking 17 Amps at 600V will be a lot more than when it switches the load at 125 VAC.

You know this already. The switch may be incapable of breaking 17 A @ 600 V even once. Or it may do it a few times before burning out. Granted that the resistive load is easy to break when comared to an inductive one, it is not wise to risk it. Doesn't the manufacturer give ratings for resistive and inductive loads separately? As per IEC, he should...

Dear Shridgar & Rakesh,

Thank you very much for the feed back. You are right that it may not be worth risking since the end user may turn off the switch before turning down the thermostat (Which in-turn switch of a contactor in series with the switch contacts to break the load) there by causing the switch to break the current at a high voltage.

The switch has a rating of 2 HP @ 600 VAC and 1 HP @ 125 HP.

Regards,

Job.

Ah, it is a motor-duty switch. That puts a somewhat different complexion on things. Single phase i assume? Then the 17A @ 125V and 6A @ 600V are inductive ratings, in which a certain motor starting inrush is already included. So, if you are using this switch for purely resistive load, a higher current can perhaps be handled by the switch. In IEC nomenclature, the resistive switching would be called AC21 while the motor duty would be AC23. AC21 tests would be at 0.95 pf while AC23 tests are at 0.45 pf. The test current multiples are also much lower at AC21.

All in all, your switch may do a resistive 17A @ 600V in a pinch. But then, you are asking only for the purpose of general knowledge aren't you ? As you say, the max voltage is 277V at worst.

Good thread.

No doubt Power dissipation is the function of contact resistance and current but that is true once contact is made solid, And what happened before that? Spark! which is function of both voltage and current both, if you increase voltage with out trading off in current you will have same amount of spark for much longer duration, as air will breakdown much before any solid contact is made, on other hand if you increase the current with out trading off in voltage you will observe higher level of spark for same duration, in brief more is voltage longer is spark duration and if more is current higher is intensity of spark.

So-

longer is sprak period - lesser is switch life

Highre is intensirty of spark - lesser is switch life

To get the optimum life you need a trade off.

Take for instance this switch a resistor. Power rating of that resistor will be 17A x 125 VAC = 2.125kVA & 6A x 600VAC =3.6kVA.

You see here Heat dissipation of the resistor/switch is directly proportional to current, because a conductor could dissipate like ~ P= IR2

So, ok those are the ratings. Try 17A & 600 VAC : 17A x 600VAC = 10.2kVA

That's about 283% comparing to 3.6kVA & 480% comparing @ 2.125kVA rating.

I say that is over, that will be a coaled red switch until breakdown/meltdown.

Er...pardon my saying so, but when arcing is involved, it is never a simple linear equation. Very dependent on the voltage.

Just for example, can we say that the switch can do 1A @ 3600V? Not on your nelly ! The switch won't just melt, it will vaporise ... believe me, i have seen it happen in the test labs. Here this moment, gone the next

Kvsridhar,, am not really my field but common sense looking through basic equation. Yap, I agree the more the voltage the greater the pressure, the higher the current the greater the heat. As for the instance, this guy might be testing both current and voltage at critical, probably, i just pray when it vaporized it will not be directed on his face. That's one destructive testing you got there, hahaha

It is quite frightening when switchgear vaporises when failing. Most tests are at 415V, 50kA level. When it clears the fault, very nice, not much to see. When it fails....wow !

Hi Noudge,

Thanks for the warning. Just to let you know that "this guy" happens to be an engineer who has a good sense not to apply a higher voltage than the switch is rated for. For your info, my applied voltage is only 277 max, and you do dot have to worry about my face being charred.

Thanks.

Hi Noudge,

I do not quite understand. Let me rephrase my question:

Suppose my loads are 7.353 ohms with 125 VAC and 35.294 ohms with 600VAC. In both cases the switch will be carrying 17 Amps. Now suppose that the contact resistance of the switch is 10 milli ohm. The power dissipated in both cases will be 2.89 watts.

As far as the switch is concerned, the flow is maintained at 17 amps and does not know whether the supply voltage is 100 V or 1000 V. Am I wrong in saying that it will make a difference only when the switch makes or breaks (As long as the voltage is not high enough to cause a break-down).

Regards,

Job.

I'm sorry zodiac, but rephrasing your problem, gets more complicated to understand.

The current is load dependent, only the voltage is made constant in all cases as should be. That is why if you try to look at how an extension chord operates - it connect all the load parallel(same voltage) only that current increased in proportion to the ratings of the load connected.

Help everyone, where are you?

You are quite right. The current is dependent on the load impedance, the switch should have negligible impedance. So, the heat developed in the switch is a function of its own resistance, and the voltage has nothing to do with it.

However, the moment the switch is operated on load, an arc develops, whose intensity is very much dependent on the voltage. The higher the voltage, the more severe the arc.

The switch is meant to (a) make, (b) carry and (c) break the current in normal and some abnormal conditions. The least of these is declared as its rating, giving current and voltage, since the rating depends on both when breaking.

Heat is what will cause the switch to fail. It is designed to handle the heat created by running 17 amps through it when closed voltage won't have any play because that equals i*r across the contacts, but as the internal resistance increases (in disconnect the resistance transitions from 10 milli ohms to the resistance of the air gap) then your heat disipation requirement comes into play. Your not worried about power dissipation in your load but, in your switch i^2*r (17*17*0.01=2.89W) becomes V*A (277*17=4709W)! Big difference!

Equipment of all types have ratings for reasons that may not be evident to the casual observer. Trying to apply a lower rated item in a higher classification is never a good idea, do so at your own risk and peril.

You get a GA vote from me!

As an inspector , I must point out that all codes and standards call for the use of equipment that is suitable to the conditions of use. The use of a 125 volt switch on a 600 volt system would clearly fall outside of what is considered to be a "code compliant" solution.

Should failure of this device cause a loss of a structure, don't be surprised if an insurance carrier were to dis-allow a claim because of the use of equipment not suitable to the requirements of the system.

Get a proper switch...

Seen some Eastern manufacturers swich 25A+ with such a device no prob.

Thing is, we can do what ever we like.....!

But does it make sense....?

Just remember to fully explain your part in its destruction when you take the switch back for a Warranty Claim.

I mean. If it is a Life threatening situation where if the loss ect........

Otherwise....... Why even bother.

Cheers.

Mark N.

Well Mark, i don't think that they would bother taking the switch back for a warranty claim, If it was me me I wouldent bother.

But what concerns me more is the problems down stream, what happens if the switch stays on when switched off, could it cause problems to the equipment, or could it be a danger to the operator's.

Always install the correct switch for the application!

Regards

Joe

Switch fault can cause serious problems at times but it all depends on where you are using electromechanical switch and how critical is the ON/OFF time for your application.

There are some solutions that may also help.

Use two switches in series in place of one and if one get locked out, use the other one to open the circuit.

Detect switch closure by monitoring signal across the switch contact. You can also monitor load power or sense current through switch contacts.

For medical applications or for avionics, one have to play very safe else may result into fatal outcome. There are good chances for it to happen. In Nuclear Reactors, three levels of safety locks are used and even then things can go wrong.

It will get welded quickly to the contact. Remember that spot welder in which you give large current pulses to weld and few pulses can not do the job. Life of the electromagnetic switch comes down sharply if you use for high current at high voltages. It is inversely proportional to the applied current through contacts. More current, less life.

Dear Dr. Shyam,

Thank you for the feed back. "Life of the electromagnetic switch comes down sharply if you use for high current at high voltages." Please correct me if I am wrong. KVShridhar has indicated "the switch is meant to (a) make, (b) carry and (c) break the current in normal and some abnormal conditions", What effects the life of the switch adversely is the arcing which takes place when the contacts make and break. The contacts tend to weld together when they make due to contact bounce. I may be wrong in saying that if you eliminate contact bounce of a switch, the chances of welding the contacts together is slim. 600 V may not be sufficient to draw an arc when making but once an arc is drawn when the contacts start to open, a few volts will be sufficient to maintain the arc. After the make, if there is contact bounce, an arc will be drawn. Given that the voltage is 600, the arc drawn and sustained when breaking 17 amperes will be a lot more than that of when breaking 6 amperes. This may cause the contacts to weld sooner if 17 amperes at 600 V than 6 amps at 600 V since the current is about 3 times. I may be wrong in saying that breaking 17 ampere at 600 V will not cause the contacts to be welded but will burn out sooner than if it was used to break 6 amperes. A switch specified for 6,000 operations when used to break 6 amps may only be giving 600 operations if it is used to break 17 amperes at 600 VAC (849 Vpk).

Dr. Shyam, What I am trying to say is a carry current of 17 ampere will not degrade the life of the switch if the service voltage is 600 instead of 125. The life comes down sharply not due to the increased carry current but due to the arc generated at the time of make and break.

Arcing due to contact bounce may cause the contacts to weld while the arcing at break will cause the contacts to erode. Please correct me if I am wrong.

Just another thing. When you spot weld any thing together, the resistance between the work pieces must be greater than the contact resistance between the electrodes and the work pieces to generate heat and fuse the metals together. In our case, we are talking about carry current of 17 amperes and once seated together, the contact resistance will not be large enough to generate enough heat to fuse the contacts together. Once seated together properly, 100 amp pulses may not be high enough to weld the contacts together.

Regards,

Job thykkoottathil.

Voltage and power rating go hands in gloves. Above to it there are limitations like dielectric breakdown for which relays re rated.

Each relay shows a curve for watts and switch life for given voltage.

Dear Dr. Shyam,

I think that I have mentioned break down earlier in "8" -- "Am I wrong in saying that it will make a difference only when the switch makes or breaks (As long as the voltage is not high enough to cause a break-down)."

Regards,

Job Thykkoottathil.

It is not true.

There is this contact resistance comes into picture in ON condition. This resistance initially is low and increases on heating and oxidation of contacts. Contact pressure also changes on heating and use. More contact resistance means more heat generated in contacts.

Some switches are designed for very low contact resistance and can take high temperature due to special material plating. These also are filled with inert gases to let it be free from oxidation.

There is another problem from mechanical motion and first contact that makes and breaks the contact surface gets affected. It is due to high electric field when contact is near and spark causes accelerated electrons to hit the contacts and etch them out.

I use Coto Relays as they are almost billion contact rated and yet they do fail. Some fail on statistical chances so many are tested at a time and for lots of time for make and break contacts. Some vacuum relays from Tyco and others may also be good.

If you operate one contact per second for a relay and if it is rated for 1 billion contact switching then also it will last only 12 days.

Please realize that manufacturer has not only given you voltage and current ratings but has also provided the minimum permissible switching operations at full rated current through the service life.

The effect of deviation from 6 Amps to 17 Amps is to reduce the number of switching operations from the original specified by the manufacturer which can be assessed by the thumb rule

(6/17) (switching operations permissible)

Hi A.A.Khi,

"The effect of deviation from 6 Amps to 17 Amps is to reduce the number of switching operations from the original specified by the manufacturer which can be assessed by the thumb rule". You are right. But if you inhibit the arc at the time of make and break, you can not only extend the electrical life of the switch (Or relay) equal to the mechanical life of the device. A well designed 5 Ampere relay can be used in applications up to 50 amperes, that is as high as up to ten times its rated current, if the arc is inhibited by a triac connected in parallel with the device.

Regards,

Job Thykkoottathil.

i urge you to learn a little more on contact systems, arcing, bounce, so on... there are far too many documents on the web, my recommendation at this point of time is this one ...though this talks about relays and your application is a power switch, the basics are there.

http://relays.tycoelectronics.com/schrack/pdf/C0_v4bg_4.pdf

Hi Shridhar,

Thank you very much for sending the link. You are very helpful as ever been. Dr. Shyam's comments were also helpful. As per your instructions I surfed through the link and also contacted the manufacturer of the switch that I am using and following was the feed back.

Thanks and regards,

Job Thykkoottathil.

The following information has been submitted :


General Comments: Hi,
If I used your HL A 27 series switch to break a 15 ampere load at 277 VAC, heat is the number of operation that I can get out of it before it fails.

Thanks,

Job Thykkoottathil.


My application is: Switching resistive loads with a small inductive component.

Found Carling switch by: Past or Current Use

Name: Job Thykkoottathil

Title: C. E. O.

Company: Zodiac Thermostats Ltd

UL testing requires 6,000 operations at full load, so it had to do 12A 250 VAC.
I do not know what 15A @ 277 VAC would do, but if it's a resistive load it should do well.

Paul Kramarz
Applications Engineer

Carling Technologies, Inc.
The manufacturer of Carling switch brand products
60 Johnson Ave.

Plainville, Ct. 06062-1177

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Glad to have been of some help. Good luck.