internal arc withstand

[rakeshNag123]

Can we specify an internal arc withstand value of say for example 85 KA for 0.4 sec while the short circuit withstand specified for the assembly is 50 KA for 1 second . I assume yes we can if not can someone explain why not ?

[PascalL]

The short-circuit withstand indicated in your example 50KA/1s is linked to the maximum short-circuit of the main supply generally corresponding to the Icc of the MV/LV transformer.

If you want to specify an internal arc performance, you can specify:

• IP arc minimum 50KA/1s ; assumption the internal arc isn’t interrupted by any SCPD (short-circuit protective device)
• IPc arc 50KA, assumption the internal arc is interrupted by a SCPD

This specification need to be referred to the IEC TR 61641 and the assembly shall be tested according to this TR.

In your example 50KA/1s, the assembly Prisma from Schneider-Electric has been tested to IPc arc 50KA and so in accordance to the requirement.

The assembly OKKEN from Schneider-Electric has been tested to IP arc 100KA/1s and so in accordance and exceeding to the requirement.

Hoping this answer will help you, with my best regards,

Pascal LEPRETRE

[rakeshNag123]

Thank you pascal !

I am still not very clear . I would want to reposition my query in this manner .

Suppose I am a specifier . I have  designed a system where maximum short circuit possible is 50 KA and I mention in the specification that the LV assembly shall have a short circuit withstand capacity of 50 KA / 1sec . Now I wish to ensure that the LV assembly shall  also be internal arc tested as per IEC /TR 61641 . So I go ahead and I mention in the specification that LV assembly shall be internal arc tested as per IEC/TR 61641 for a value of 85KA/0.4 sec or 100 KA/0.3 sec .

Meaning when system short circuit level is 50 KA can I demand my assembly to have a internal arc withstand of 85 KA/0. 4 sec or 100 KA /0.3 sec . Is this a logical spec in ?

[PascalL]

I  try to be more precise,

Regarding the level of short-circuit, as you said the maximum prospective short-circuit is 50KA. So, using your wording it is not logical to require an internal arc withstand exceeding this value but it will propose a safety margin.

About the duration now, you can specify a duration of 0,3s or 0,4s in that situation you need to be sure the time delay of the Incomer (if there is one) don’t exceed this time.

You minimum specification could be  for internal-arc 50 KA/0,4s or 50 KA/0,3s and if you want a have a safety margin to that performance also 85 or 100 KA/0,4s

[rakeshNag123]

Hi pascal ,

Thank you for your explanation. I must admit you have addressed my query precisely . In order to  assure myself that i have understood , wish to take your final opinion again along with an additional query . 

You have mentioned in your response below  "You minimum specification could be  for internal-arc 50 KA/0,4s or 50 KA/0,3s and if you want a have a safety margin to that performance also 85 or 100 KA/0,4s"

so , this means minimum i may specify  is 50 KA /0.4 or 50 KA/0.3 (if prospective maximum short circuit is 50 KA /1s)but for building up a safety margin i can specify 65 KA/.5 or 85 KA/0.4 or 100 KA/0.4 . And i may give reasoning to the customer that higher the internal arc withstand of a panel more safer is the panel ( irrespective of the maximum prospective  short circuit current). Am i right here ?

However consultants are questioning on the building up of safety margin and ask under what circumstances can a current of 65 KA , 85 KA or 100 KA might flow in the system when maximum prospective short circuit current is 50 KA only ? 

[PascalL · Accepted solution]

Hi Rakesh,

Regarding your reply: "i may specify  is 50 KA /0.4 or 50 KA/0.3 (if prospective maximum short circuit is 50 KA /1s)but for building up a safety margin i can specify 65 KA/.5 or 85 KA/0.4 or 100 KA/0.4 . And i may give reasoning to the customer that higher the internal arc withstand of a panel more safer is the panel ( irrespective of the maximum prospective  short circuit current). Am i right here ?"

Yes you are right Rakesh.

About the second part of your reply: "under what circumstances can a current of 65 KA , 85 KA or 100 KA might flow in the system when maximum prospective short circuit current is 50 KA only ? "

If the Icc is 50KA you have no chance to have 65 or 85 or even 100KA.

If I make a parallels explanation...suppose you buy a car. The car is able to run at 190km/h. The speed is limited at 130km/h on the highway. So do you have any chance to exceed 130km/h? No but it is comfortable, the result is when you drive the noise will be lower, the reliability higher, the safety also.

To an assembly it is the same thing. Probably in case of internal arc the impact into the assembly will be less important, probably the MTTR(mind time to repair) will be better...we call that a safety margin.

Who can do more, can do less 🙂

Best regards,

PascalL.

[rakeshNag123]

Dear Pascal 

I am truly Delighted with the support extended and your  patient response to my queries . Will be glad if you can share any related literature for my reference and use ( if you already have any) 😊 and once again thank you ! 🙏