Switchgear rating

[pkinton]

If I have a switchgear lineup with two buses and a tie with each bus rated for 65kA. With the tie breaker open my available fault current is approx. 43kA, which is acceptable per NEC. If I close the tie breaker (both mains closed) my available fault current on either bus is approx. 82kA. If an automatic tripping scheme is in place where a main breaker automatically trips some short time after the tie is closed, is the exceedance of the switchgear rating acceptable per NEC? What if the scheme is not automatic but manually performed?

Thanks,
Paul

 

[ron]

Re: Switchgear rating

What is a short time later, 100mS?
Since you describe a make before break scenario, of course the two sources are synchronized (or checked for sync), right?
If the tripping is manually performed, what is the time in which the two sources are paralleled.

I very often am involved in design of systems that have a 100m Sec. overlap and longer.
The answer is, it depends. Tell us more about the system.

 

[pkinton]

Re: Switchgear rating

The system has been phased and is ok.

The automatic time would be settable but is more of a hypothetical question. But for arguments sake lets say 30 cycles.

For the manual operation say a couple of seconds (less than five seconds) for an operator to close the tie breaker then open a main.

The questions are partially hypothetical as I see this type of application and haven't been able to find a specific answer in the code books. But the driving situation is an existing chemical plant where we were asked to perform a short circuit study based on current conditions as the system has greatly changed since its initial installation in the late sixties early seventies. The system has two 13.8kV feeds into two 13.8kV/480V transformers. This is actually just one of many substations at the plant. Operation of the breakers is performed locally with control switches (the doors on the switchgear are closed).

While this question is based on a 480V system my question still applies for medium voltage systems too. What considerations are there?

 

[wanderer20001us]

Re: Switchgear rating

In the DTE utility area, this is somewhat common, both at the 480V and 13.2kV/13.8kVlevels. In most cases, the switchgear is set up for automatic closed operation or open transition manual operation for customers. The automatic closed trasition uses the 'a' contact on the main to trip the tie, threrfore there is no intentional delay of the overlap.

DTE will perform controlled manual closed transition but only when their substations are tied over to the same transformer. Then you are paralleing the same source and the fault current is not doubled.

If you are manually transfering without the utlity's involvement, there is the risk of a failure while you are switching that would generate fault currents in excess of the switchgear's rating.

You will need to check with the AHJ and the utility to make aure typical operating modes are considered and risks evaluated.

my 2 cents

 

[ron]

Re: Switchgear rating

First my opinion.... Logic indicates that during closed transition for any length of time, there is a possibility of a fault during that period, 6 cycles, 30 cycles, 5 seconds et al. The SWGR should be designed to withstand and interrupt that amount of current coming from both sources.

Then there is the other opinion ..... There is no code or standard that specifically addresses this condition. I argue with my colleagues constantly regarding this issue, as it is almost always an issue, whether it is two utility sources, or the utility and the 'house' generators. Many of my colleagues feel that if it is less than a few seconds, not to consider that case. Some of my other colleagues agree with me.

The applicable code is NEC 110.9. It states that the gear should have a rating sufficient for the nominal circuit voltage and current on the line terminals of the equipment. So the question becomes, what is the available fault current at the line terminals of the equipment? If the fault occurs upstream of the SWGR, then the fault current at the line terminals will be equal to one source's contribution plus any building motor load. If the fault occurs at the tie breaker (or further downstream), the fault current at either line terminal will be equal to one source's contribution.
The catch is what if there is a fault on the line terminal of the equipment. Then you will have both sources and building's motors contributing. What is the likelihood that a fault will occur at the line terminal/bus, or during a few second closed transition?
It is for an engineer/designer to ponder the possibilities.
That's why there is judgment involved in all aspects of this 'stuff'.