Motor terminal box fault withstand capability. 13.2kV motors. 1

[qqitek]

Dear Folks,
I got confused by API 546 cl. 3.1.2 requirement on motor terminal box (MTB) with rupture discs, which states:
"The terminal box for the main power lead terminations shall be capable of withstanding the pressure build-up
resulting from a three phase fault of the specified MVA (one-half cycle after fault inception) for a duration of 0.1 sec.
[...]
If a rupture disc is used to relieve pressure build-up, it shall not compromise the environmental rating of the box and the discharge from the pressure release shall be
directed away from locations where personnel may be normally present."

Now question - how do you understand WITHSTANDING?
a) Does it allow that terminals, accessories, connections, CT's, surge arresters, etc. can be burned, melted, vaporized, etc.... simply damaged or destroyed, while MTB box and it's mechanical structure shall maintain it's integrity without damage?
or
b) MTB with all accessories shall survive the max specified fault WITHOUT any damage?
or
c) both MTB with all accessories will be damaged, but MTB will not explode thanks to rupture discs, thus no harm to personnel.

Vendor is following c) approach, and it seems to me not really complying API.
I would appreciate any experience/hints, or even clarification (as there might be language issue ;-)

Best regards
qqitek

 

[LionelHutz]

C sounds right. The box can distort and bend during testing and still be found acceptable.

 

[waross]

CT's, surge arresters, etc. can be burned, melted, vaporized,

"The terminal box for the main power lead terminations shall be capable of withstanding the pressure build-up

Pressure build-up does not cause burning, melting nor vaporizing.

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

 

[LionelHutz]

Burning, melting and vaporizing of internal components does cause a pressure build-up.

 

[qqitek]

indeed, arc during fault will heat all up, plasma created might be very hot, pressure increases and rupture discs opens to prevent explosion, i think.
But still - whether API requires to survive all internals, or not?
Or we need to agree that all will be wasted, inclusive terminal box?

qqitek

 

[7anoter4]

Since it is not possible to seal hermetically the box, in my opinion, an explosion in a terminal box it could happened not only in case of arcing but also if the hazardous gas from an environmental hazardous location penetrates the box and exploded in interior due arcing, overheating and other. The flame is laminated through the lid and box gap and is already cold enough exiting the box and it does not produce another explosion outside the box. The box lid has to stay in place then all the time not important what it happened in the box.

 

[LionelHutz]

It is a fabricated box or a cast box? A fabricated box could distort and still be deemed acceptable. A cast one not so much.

 

[qqitek]

Hi Guys,
we had long discuss with Vendor, and apparently they treat motor terminal box similar to the MV SWGR, specifically IEC 62217-200 and AFLR internal arc classification.
If you read al 4 criterions, after the arc test, external structure and enclosure is allowed to be deformed, while standard is mute on internals. SO i can draw conclusions, that there is no requirements to internals and they can be melted/burned/evaporated.
However I am not inclined to accept mixing standards from API world with those from IEC, and comparing motor terminal box with MV SWGR.
At the end they are different devices.



rgds
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