Tuesday, February 15, 2011

Medium Voltage Distribution: Deadbreak Elbow vs. Load break Elbow

As with all construction work and within life itself, safety should come first.  So remember safety in everything you do!

As you may or may not know, these devices are seperable splices and elbows used in the underground medium voltage electrical distribution industry to attach cables to equipment (such as a transformer or switch) and to attach cables together (splice) in man-holes.  A dead-break elbow requires the cable to be “dead” in order to be detached or attached and is typically rated for 600A.  A load break elbow can be attached or detached live or under “load” and is typically rated for 200A.  As you can see, one difference between them is the increased current rating of a dead break elbow, but there are various other differences as well.  For more information on the difference between this type of connection vs. the traditional splice, check back for the topic: Medium Voltage:  Hammerhead Splice vs. Traditional Splices.

 
Load Break Elbows:

A load break when used with a hot stick can be connected or disconnected under load, but can only be used with up to a 250kcmil cable.  At 100% insulation level, a 250kcmil is good for 345A in underground duct, so you would be de-rating your system anyway by using a 200A elbow with a 250kcmil cable, but they are still widely used due to the flexibility given to the distribution system at the equipment. 

Think about it…if you had a critical application where power disruptions were extremely costly and maintenance was required on up-stream equipment in, let’s say a non critical area, this product would be great.  You could disconnect the load without disturbing the critical portions, perform the maintenance and then reconnect it.  Just make sure to take safety precautions, making and breaking electrical connections under load is very dangerous.

The load break elbow is typically not used in man-holes, because it does not allow expansion like the dead break elbow.  The construction of the load break elbow does not allow it to be connected to other elbows, but the dead break does.

Dead Break Elbows:

A dead break or “hammerhead” can only be disconnected when not under load, this is because you need to actually unscrew a pin to detach it.  It would be pretty tough to unscrew the pin with a hot stick and the arc from the disconnected load, if under load, would be pretty substantial. 

A dead break allows an increased current rating (up to 600A) and can be used on basically all cable types.  At 100% insulation level a 750kcmil cable is rated 610A in underground duct.  When would you use a 750kcmil? In most non-utility cases you would design or install sets of 500kcmil or smaller if you needed more than one 500kcmil could handle.  Even from a 1200A breaker, two sets of 500kcmil per phase would be sufficient for distribution (about 1000A), if the load was even that high.  Most of the time you set the relays down to the actual load in order to limit the copper used.

A dead break when used in man-holes for splicing cables allows the flexibility of connecting more than two cables together as well as future expansion if required.  Say another building was added that needed power, you could just connect that building to the existing array of dead break elbows.  This also works great when temporary power is needed for construction or even if the circus is in town and they need medium voltage (it happens!).  They can pick up power from the man-hole with the dead break elbows or from a piece of equipment with dead break elbows.  I’ve heard that ships are now starting to use medium voltage shore power, I wonder what kind of connections they use. 

A dead break still provides flexibility during maintenance it just requires a brief outage to allow the splice to be broken.  This outage could be scheduled at an off-peak time or after ensuring that all back-up systems were already running and/or transferred to another source such as a generator.  I wouldn’t trust a UPS alone in this case unless it was connected to a second source of power like a generator.  The amount of time to disconnect the splice and coordinate the outage would cut it real close.

Deadbreak elbows are fairly large, so make sure to check the space restraints of the location you are installing them.  If you do run into a space problem, their are various other seperable splices on the market that may be suitable, check with the manufacturers.

Although I prefer Richards Manufacturing for these products (just my preference) they are also manufactured by ABB and Thomas and Betts (Elastimold) and they can be ordered with all sorts of accessories.  
Keywords: Hammerhead Splice, Dead Break Elbow, Load Break Elbow, Richards Manufacturing, Seperable Splices, Temporary medium voltage connections

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