Purpose of Double Manhole Lid

[655321]

There is an electrical manhole with a cover frame designed for an inner and outer solid cover, one below the other. The inner cover is locking. Does anyone know what the purpose of using the double manholes?

 

[bimr]

You have not provided m uch detail on the manhole, for instance the manhole depth and voltage. Here is a link to an article that decribes the requirements for electrical manholes:

http://ecmweb.com/mag/electric_manholes_nec_say/

 

[655321]

There's actually a number of manholes of varying size and depth containing a number of different power and control cables, they're located around a power plant. The old drawings show a detail for the ring and "double" cover (an inner cover and outer cover). The detail references a "Flockhart Foundry" catalog number, but Flockhart Foundry apparently no longer exists...the drawings I'm looking at are 50 years old.

The ring and cover are similar to a Neenah Foundry R1755, cutsheet attached. So is the purpose of the double lid protect from frost?

I've never seen these types of frame and covers used before. Any insight into when and why they are used would be great. Any one have an old flockhart catalog?

Thanks.

 

[bimr]

Would appear to be a watertight and locking cover over an electrical junction box or splice box. Here are some examples of these items:

http://www.romanstoneco.com/manholes.html

http://www.highlineproducts.com/fpc.htm

http://www.pge.com/includes/docs/pd...tion/greenbook/servicerequirements/028028.pdf

The electrical equipment is probably unique and specified by the local utility.

 

[655321]

Thanks Bimr,

I do know exactly whats under the lids, deep concrete vaults containing cables and splices and sump pumps. See, what I'm trying to figure out is why the original designer used double manhole lids. You can order a watertight and locking frame with a single cover. After doing more looking around the web I think the reason behind using a double lid arrangement is to provide frost protection. I would like to confirm this and get a little more insight into when and why they are used as opposed to single covers. Like I said I've never seen these used before.

Thanks again.

 

[655321]

According "The Electrical world, Volume 32, page 72", dated 1898 (found on Google books), Its not practical to seal manhole lids watertight when they are exposed to vibration and impacts of street traffic. So the outer manhole is intended to take the impacts of traffic, while the inner manhole lid is gasket-ed and makes the watertight seal.

So is this still the practice today, are double manholes used for this reason?

 

[bimr]

Don't believe that it is the practice at the present time.

The original concept with the watertight lid was probably an attempt to keep water out of the manhole. Water and particularly salt water in the winter from salting streets create an extremely corrosive mixture and causes electrical failures.

New York city started installing vented vaults because there may a buildup of explosive gases in vaults.

Here is a link to a vault specification. It seems the present standard is a vented vault with sump pump.

http://www.uky.edu/Facilities/CPMD/standards/div2/02605S01.pdf

Link to a discussion of fire fighting issues in vaults.

http://lafdtraining.org/btrm/wp-content/uploads/2008/05/manhole_fires.pdf

Don't believe that there is any universal standard for manhole castings. Each utility seems to have their own standard:

http://www.alhambrafoundry.com/Alhambra_Foundry_Catalog.pdf