Reason I don't access Electric Manholes

[sprayandpray]

http://www.wimp.com/explodingmanhole/

[Mke]

EeeeeK!

but don't worry, this never....errrr.... Hardly ever happens.

Great find Spray

mke

[RD_Wrangler]

Looks like a rat chewed through the wrong insulation!

[headcipher]

We have an old network down town that is so old that it has no switches or breakers in it the entire way back to the plant. I know that someday that same thing will happen, except it'll burn for blocks before the plant can shut it down.

[ProfessionalLocator]

We have an old network down town that is so old that it has no switches or breakers in it the entire way back to the plant. I know that someday that same thing will happen, except it'll burn for blocks before the plant can shut it down.

In an urban distribution system a "network" is a group of high voltage feeders out of the same substation whose transformer secondaries are all tied together. This way if a transformer, or feeder, goes out the customers stay on becasue they are not fed by an individual transformer.

The transformer has primary and secondary side "interface" boxes. The primary side is a simple switch to connect, disconnect the primary to the transformer. The secondary side also has a similar switch arrangement. There is also provision to ground through the switches without shorting out either the primary or secondary feeds. There is also a sensor in the secondary that recognizes if there is a back feed from the secondary to the primary side which will cause the switch to open. The primary side is called the switch`` and the secondary side is call the protector.

There are heavy duty fuses in the secondary side switch and these are in some places referred to as "catastrophe" fuses. They are high rated so that if a secondary cable shorts a 250 mcm cable will burn in two, burn itself clear, rather than shut down service. Since the secondaries are all tied together a burned open secondary will simply be fed from both directions leaving the secondary network supplying the customers. Of course if the secondary that burns open is for a service that customer will be out of service.

The fire and arcing you saw in the manhole looked like such an event, secondaries burning open.

There is another hazard associated with this type of system which can also affect normal secondary distribution systems. If the cables are overloaded and heat up it cause the insulation to bake out. This releases hydrocarbon gases which are flammable and they can ignite into an explosion.


I am comfortable entering these manholes, have been in countless numbers of them, and they are nice in Winter as they tend to build up heat.
In Summer they are often ovens.