Purpose of Double Manhole Lid
Purpose of Double Manhole Lid
(OP)
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?





RE: Purpose of Double Manhole Lid
http://ecmweb.com/mag/electric_manholes_nec_say/
RE: Purpose of Double Manhole Lid
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.
RE: Purpose of Double Manhole Lid
http://www.romanstoneco.com/manholes.html
http://www.highlineproducts.com/fpc.htm
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The electrical equipment is probably unique and specified by the local utility.
RE: Purpose of Double Manhole Lid
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.
RE: Purpose of Double Manhole Lid
So is this still the practice today, are double manholes used for this reason?
RE: Purpose of Double Manhole Lid
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.
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Link to a discussion of fire fighting issues in vaults.
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Don't believe that there is any universal standard for manhole castings. Each utility seems to have their own standard:
http://ww