MV Switchgear Short Circuit rating for 3s

[Jay08]

Why do MV Switchgear are rated for short circuit rating 3s?
I have seen (limited to my experience) tripping time in relays is within 1S for faults. Overcurrent it may be more based on current value, but that doesn't a concern to SC rating.
Even if I consider NGR current limiting to much lower value (let's say few hundred amps) system earth fault, in general relays are set to trip within 1s if not instantaneously.

Please excuse my ignorance if I missed any silly point.

 

[waross]

It has to do with the damage curve.
Heat/temperature builds up rapidly under a short circuit.
1 sec tripping for 3 sec rated equipment gives a good safety factor.
Hopefully, a 1 second trip will ignore switching transients but get equipment off-line before damage occurs due to excess temperature.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

In the event that the primary relay/breaker fails to clear a fault, backup protection can take much, much longer to clear. Backup protection times of more than 1 second sometimes occur.

 

[Jay08]

Thank you waross & bacon4life for your input.

Waross, Since it is "SC" rating that has been rated for 3s for mv switchgear, i believe transients are not much of a concern here. So far i have seen for any sc(phase/earth) which is quite a high value 50P is set to trip instantaneously, in some cases earth fault with lower value with some delay but with very minimal delay not even 1s.
I am trying to understand the idea of rating mv switchgear for 3s, where all other switchgears (even HV) are rated for 1s and it is enough. Like you said safety margin could be consideration but again what mv has to do with to specify rating for 3s, where in rest of the cases it has to be defined for 1s.

I am attaching image from SLD for HV and MV example.

Bacon4life, With my limited experience i have not seen backup protection (upstream relay) go beyond 1s trip for sc values. Even power transformers inrush current at mv level is 5-6 cycles. I am attaching one example of relay setting.
I am sure you have seen cases where tripping time going beyond 1s, in case you have any reference document please do share.

 

[bacon4life]

Many kinds of IEEE rated equipment use 2 seconds for the thermal withstand period. It looks like you might be following IEC standards, and I am not familiar with whether different kinds of equipment might have different preferred ratings in the IEC standards.

The fault withstand of a device can be limited by either the peak short circuit forces or by the thermal heating of the conductor. The manufacturer might then round down to label the with equipment with the a preferred rating class (i.e. 10 kA, 40 kA, or 63 kA). In one case I had piece of equipment located where the fault current just barely exceeded the nameplated fault rating. The vendor was able reanalyze the equipment for me and provide a slightly higher kA rating with a somewhat shorter duration.

Most of the instances where I have seen clearing times exceed 1 second tend to be at fairly low current levels or for very extreme contingencies. For example, the loss of a substation battery requires remote clearing of all transmission terminals. The clearing ends up being sequential since the weaker terminals cannot detect the fault until the stronger terminals clear.

A recent example of an actual fault taking more than 1 second to clear was a squirrel started fault on a Normally-Open switch between two circuits.
Part way through the fault the arc flashed over to involve the second circuit, which resulted in fault current splitting between two feeder PCBs and dramatically slowing down the relaying.

 

[che12345]

Jay08 (Electrical)(OP)25 Jul 23 09:47
"...Why do MV Switchgear are rated for short circuit rating 3s?...."
There were numerous valuable advice posted regarding rated short-time withstand current 1s or 3s. I only wish to point out other historical reasons which is not mentioned in the posts.
1. In the past, all major standards BS, VDE, IEC, IEEE specified breakers rated short-time withstand current test to 3s..
2. Recent IEC, IEEE... etc do not stated that it Shall be tested to 1s only. It is open up to the manufacturer, 1s or 3s; or even any value between 1-3s. However, test at lower than 1s is also allowed but with limitations etc....,and work out the value based on the similar Tested device.
Che Kuan Yau (Singapore)