I2T - 31,5kA/1s x 31,5kA/3s

[rudynick]

Hi there.

I got a disconnector switch with short current rated to 31,5KA/1s, I need to demonstrate that his contact are able to support 31,5kA/3s. Based on type tests recorded earlier, the only information I have are the contact resistance and contact area, can anybody give me a tip if it is possible to "SIMULATE" this extra thermal stress? I really don´t think that i²T is saying anything about this extra contact thermal efforts.

Best regards;

Manoel.

 

[cuky2000]

What is the system operating voltage?

Beware that HV disconnect switches are not rated for interrupt current. The main function of HV disconnect switches are for isolation and should be operated after the circuit breaker is opened.

 

[rudynick]

Oh yes, sorry for that.

145KV is the sytem voltage, the 31,5KA is the short circuit withstand current, of course the DS will not OPEN with this current.

 

[Skogsgurra]

There are at least two limits; dynamic force limit and thermal limit.

If the limit is given for one second, it is probably a thermal limit. So you shall look at temperature after three seconds instead of one second.

That alone says that the disconnet can not carry 31.5 kA for three seconds. Temperature rise (T) is proportional to power times time T=I^2*R*t (actually, R is not constant - but increases with T) so you needn't do any complex calculations using contact resistance or contact area and resistivity.

In my view, it can't be done.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rudynick]

Yes, I think that can be done too, just checking..

Thanks.

 

[Skogsgurra]

I said 'It can NOT be done'.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Marmite]

If the disconnector is rated at 31kA for 1 second, and that's a thermal limit then the rating will be reduced to something like 18kA for 3 seconds. Common sense would tell you it's going to be a reduced rating. You would need to calculate the thermally equivalent short circuit current. See Chapter 4 of the ABB switchgear manual for more guidance.
Regards
Marmite

 

[rudynick]

This is exactly the point Marmite, I suspect that we are far away from the limit, I think I can easily pass 31,5kA/3s without any problem.

I trying to figure out how I can evaluate with the data I have to give some degree of thrust that our DS will hold 31,5ka/3s.

 

[rbulsara]

If the switch's published/listed rating is 31.5 kA for 1 sec, that is what it is. It does not matter what you can 'demonstrate' or prove. At least not in the USA.



Rafiq Bulsara

http://www.srengineersct.com

 

[Skogsgurra]

rudynick

Do you understand what we are saying? There's no way you can do that!

Why do you so desperately need to 'prove' something that cannot be proved?

Beware. There is legislation. You will be sorry if you carry on with this.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rudynick]

STOP RIGHT THERE…….

I am not so desperately to prove anything.... and for sure, if we will export to US, be sure that TOSHIBA will provide the needed equipment according with your requirements ok?. No issues about that.

The problem is that, for some disconnector switches, rated to 31,5KA/1s, we made recently the tests for 40kA/3s, and the DS had passed gratefully on this much more severe condition. I remember that the model was different from this one in case. The first, we discuss a central brake, the second was a vertical opening.

What I trying to do is a mathematical approaching to this one central opening DS, just to check if I need to improve something in the contacts sets, in other to guarantee that the disconnector switch will, for sure, passes the short circuit test with 31,5/3s.

The two types of disconnector switches are totally different, but the contacts area, contact type and others things are really very similar, that’s why I trying to do this approach.

This shows that, the equipment is fairly away from the thermal limits, and this is exactly what I trying to discover, what is the thermal limit of our equipments, without expensive tests, like short circuit type tests ok?

Regards;

Manoel.

 

[Skogsgurra]

You have been warned.

What does the manufacturer say? He has done all the testing.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rudynick]

I am the manufacturer.

I work at TOSHIBA.

See, I think I could not make myself clearly, sorry for that.

I remember that I trying to receive some help, not to fight or been “warning” for some reason.

Please let it go,

Thanks for all.

Regards.

 

[waross]

If a switch is rated for a specific fault current for a specific time, most of us assume that it has been tested by the manufacturer and those ratings are the result of the testing.

I realize that a switch with a higher rating may be more expensive, but the people who make and test switches know this better than I do. If it was possible for a valid test to show a higher rating for this switch, the manufacturer may list the switch for the higher rating and sell it for a lot more money.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

Sounds like this is probably built to IEC standards, but IEEE Test Code for High Voltage Air Switches C37.34-1994 calls for the short circuit test to be a minimum of 3 seconds and a maximum of 6 seconds, adjusting for the same heating equivalent. Somewhat oddly, the Standard for HV switch Preferred Ratings C37.32-2002 then states that 1, 2 or 3 seconds are acceptable preferred ratings.

The annex of C37.32-2002 provides some equations for calculating temperature rise of switch components. However it does require knowing the conductivity, cross section and maximum allowable temperature of all components of the switch. As others have stated this wouldn't let you exceed the nameplate ratings. You could use it as sanity check to see if the manufacturer might do more testing for you.

 

[scottf]

To cut to the chase, you don't have enough info to calculate the thermal current performance. You can either derate the 1 sec value to a 3 sec value or contact the manufacturer. Anything else is a waste of time.

 

[LionelHutz]

Using the contact resistance and area, combined with the characteristics of the contacts and associated conductors - mostly resistance and thermal - you should be able to calculate the temperature rise.

 

[7anoter4]

I agree with beakon4life.
Calculation of the thermal short-circuit resistance of a disconnecting switch is complicated.
For rated current only the contact temperature depends on a lot of factors as the material resistivity, force applied
on contact, material compressive strength, thermal conductivity and other[ all these depend on temperature].
In short-circuit case a repulsing force will diminish the force applied on contact and an elevated temperature could conduit to welding of contacts. If for short time [one second or less] one may consider adiabatic heating for 3 second some heat could be evacuated outside.
ANSI Standard C37.20.2-1999 specified rated short-circuit for 2 second, MOM [momentary short-circuit] for 10 cycles and peak short-circuit rating.
Rated momentary withstand current: This is the maximum rms total (asymmetrical) current that the switchgear bus must withstand, and is measured at the major peak of the maximum cycle of the maximum offset phase. The test to verify this rating requires a test duration of at least ten cycles of power frequency. During this test, the peak current at the major peak in the maximum cycle must be at least 260% of the rated short-time current of the switchgear.
Peak withstand current: This is not strictly a rating, but it is the peak current value (260%) that must be attained during the momentary withstand current test described above.
Short-time withstand current rating: This is the average symmetrical current that the switchgear must be able to withstand for a period of two seconds.
So if you know the peak withstand current you may divide by 2.6 and this is the short-time withstand current for 2 sec.
From some manufacturer's catalog the short-time withstand is stated for 3 sec. and is the same as calculated for 2 sec. according to ANSI. But no mention of 1 sec. withstand.

 

[Marmite]

In the context of Rudynicks posts this is all a pointless and futile exercise because unless the switch is tested and proven to comply with the new "calculated" enhanced rating, nobody in their right mind would buy it.
Regards
Marmite