Vacuum Contactor - Specifications 1

[charz]

Following is the technical specifications of vacuum contactor in which I have some questions.

1.Does it mean that the protective relays can be used to trip the vacuum contactor till 12,500A?, then why Maximum break capability is mentioned as 8000A?
2.What is the difference between the interrupting current and breaking current? I thought the two terms were same, but here it is mentioned with different rating
3.In Some other specification, I find "Maximum Interrupting current (3 operations)"- does it mean, the contactor can interrupt only for 3 times at the interrupting rating.What exactly does it mean?
4. What is the difference between the SC withstand rating and short time current rating capability?

 

[RRaghunath]

Item wise:
1. True, the subject contactor can interrupt safely 12.5kA and protective relays can be used to trip the contactor. As per IEC60470 for MV contactors, the requirement is 8-times the contactor continuous current rating, as a minimum.
2. Interrupting and breaking are same. Make and Break is different and more strenuous duty on the contactor as the contactor is asked to open even before the conatcts make completely, in case of make and break duty (also called CO duty).
3. Interrupting at max rated fault level is not repeateable without limits. 3-operations means the contactor may need to be inspected / tested after three break operations at max fault rating.
4. SC withstand (1/2cycle) duty is speaking of contactor capability to withstand electro-mechanical forces. Short time current rating is short time thermal rating and used in protection coordination studies.

Rompicherla Raghunath

 

[charz]

Thanks Raghun,
What is chop current? How is it measured? I understand that chopping phenomenon as forced current interruption before natural zero.

 

[RRaghunath]

In Vacuum, the current is abruptly interrupted (chopped) giving rise to overvoltage issues. This used to be a major problem in the initial days of vacuum technology. To protect the connected equipment downstream, installation of surge arresters used to be a requirement.
With the improved technology, the chopping has become a lesser problem as the arc could be maintained close to zero crossing of the current waveform.
0.5A chop current refers to the same. The manual would also explain that the surge arresters will not be required in general and also would have provided guidelines when to provide surge arresters.

Rompicherla Raghunath

 

[cherryg]

Hello Charz..regarding your first question, please note that make and break capacity defines a value of current under steady state conditions which the contactor can make/break without welding or undue erosion of contacts.The maximum short circuit current is obviously higher then this value and will be the maximum value that the contactor safely clears. Hence the difference in values.For a good discussion on vacuum chopping, please see article

http://www.joslynclark.com.cn/up_fi...ite paper(databinuser-2012-6-18-18-00-46).pdf