does anyone know the purpose of tie breakers in switchgear.

Lets say you have a 4000 amp tie breaker ( with a spring loaded handle) between two 3000 amp switchboard panels (each 3000 amp switchboard section housing about 600 amp feeder breakers. Why is this tie breaker connected via copper bussing in the switchgear necessary?

It is likely that the fault duty is too high for insulated cables to withstand without damage. Properly braced busbars may have been specified particularly for the increased fault current when the tie breaker is closed. The fault duty effectively doubles in that situation—with three breakers closed, two transformers are then operating “in parallel”.




[This message has been edited by Bjarney (edited 08-31-2003).]

Main-tie-main switchgear is very common in critical facilities. Depending on the application, each of the mains is designed to carry half of the total swgr load on a normal day. In the event of a failure of one source, the main breaker on the "bad" source is opened, then the tie is closed for the "good" main to carry the full load of the SWGR.
Sometimes I design in a main-tie-tie-main. This allows maintenance access to either side of the tie without any live bus to contend with.

In the 8 years I've been with my current company, we have used our tie breakers more than once. I think they are a good investment for a company.

We melt copper in large induction furnaces which will "freeze up" if you leave power off for too long. When a main lead blows off the 34Kv, we can tie into a secondary incoming line.

At my plant we have 6 indoor substations that we can tie together in case of a xfmr failure in one of them. We have used it a couple times here in the last few years after a xfmr failure. We also use the Kirk key sytem to avoid any kind of mix up when tying subs together.