Difference between Metal Clad & Metal Enclosed MV (6.6KV) Switchgear 2

[aysohrab]

Gentlemen, could you please advise the main differences between Metal Clad & Metal Enclosed MV (6.6KV) Switchgear? Which one is better from operation and safety aspects?

Many thanks in advance for your support.

 

[davidbeach]

It depends; define better.

 

[dpc]

"Metal-clad" switchgear refers to circuit breaker gear and "Metal-enclosed" refers to fused load-break switches, I believe. At least if you are in the US and referring to ANSI-standard equipment.

Two completely different types of equipment.

 

[dede61]

While Metal-clad is much heavier and more robust then Metal-enclosed there is a significant difference regarding shirt-circuit withstand current:
Metal-enclosed is mostle between 16 and 20kA
Metal-clad runs up to 31.5kA, 40kA and 50kA
Most of the modern Metal clad switchgear have withdrawable CB's = disconnection in air when driven out of the contacts + shutters closed.
Most Metal-enclosed have a seperate (in a smal SF6 container) integrated switchdisconnector + integrated earthing switch (this last item is not valid for all constructors some still have the earthingswitch seperatly in air). Metal-enclosed switchgear standard doesn't have a withdrawable CB (= no shutters) but I've seen some deviations on this last items (meaning I have seen some that do have withdrawable CB's).
DPC, You have Metal-Clad with fuses to as well as you have Met-enclosed with CB's its common.
The main difference lies in the short-circuit withstand current.

regards,

Danny

 

[dpc]

AFAIK, any reference to "metal-enclosed" switchgear for ANSI rated equipment refers to fused switches. Any reference to "metal-clad" refers to circuit breaker switchgear. I don't have my ANSI C37 in front of me, but I've never seen fuse switches in "metal-clad" switchgear, at least in the US.

IEC land may be different.

 

[jraef]

Withstand ratings have to do with the costruction standards asked for, not the basic design. We use Metal Enclosed FLIS (Fused Load Interruptor Switch) gear that is rated 31.5kA.

dpc's definition is correct, Metal-Clad is circuit breakers, Metal Enclosed can technically be either, but tends to be FLIS because of a key issue with vacuum circuit breakers; they cannot be used for safety disconnect without being withdrawn from the bus as in Metal Clad, or isolated with an additional switch that provides an air gap of sufficient dielectric strength.

In the event of a fault, vaporization and deposition of contact material inside the vaccum bottle can allow leakage after they interrupt. That leakage can be lethal, so when circuit breakers are used, you must have an air gap isolation for worker's safety. By withdrawing from the bus and shuttering the opening, Metal Clad gear provides that air gap without requiring an additional switch mechanism.

Metal Enclosed FLIS gear uses a Fault-Make / Load-Break rated disconnect switch that provides the necessary air gap, but short circuit protection is provided by the fuses. It tends to be much less expensive that MC gear and shallower, 36 - 44 inches compared to 84 - 96 inches for MC gear. But unlike MC gear, you can't stack one unit on top of another, so the gear length can get out of hand in large systems. If you were to use vacuum circuit breakers in Metal Enclosed gear, you must still add an isolator switch, which adds even more to the size and too much cost compared to just using Metal Clad. You used to be able to get air circuit breakers in ME design gear years ago, but I don't think anyone does that any longer.

So all the classic debate points of fuses vs circuit breakers must be decided first, then the gear you use follows that decision.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

Another factor is available space. Metal-clad switchgear requires front and back access and is about 8 feet deep. Metal-enclosed fuse switchgear requires only front access and is not nearly as deep.

And cost, of course.

 

[swgrmfg]

Furthermore...
Metalclad switchgear has metal barriers between cable and main bus compartments with no intentional openings in between (usually the equivalent of 11Ga), also barriers between cable compartments.

And to answer the original question: all else being equal in terms of good equipment design, safety primarily depends on the operator and his knowledge of the type of equipment he is dealing with.

 

[cuky2000]

Metal-clad is one type of Metal Enclosed switchgear. Below is an excerpt from the IEEE Std #141 regarding type of switchgears.

http://cuky2000.250free.com/Switchgear_Type_01.jpg