Difference between ATS & Interlock

[masalah]

Dear all,

In a Single line diagram a dotted line is drawn between the two incomings of an MDB (one coming from transformer & the other from a generator), and written above it (ELECTRICAL AND MECHANICAL INTERLOCK).
I understand that this interlock is to execute the task of (1 out of 2), but why is that used and not an automatic transfer switch (ATS)??

Cost only? easier in installation?? what are the advantages of using an ATS over this scheme??

Thank you in advance.

 

[waross]

You may find the same notation on an ATS. Your equipment may function as an integrated ATS

Bill
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"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

An ATS has proven technology and software for:
monitoring the incoming power,
determining when there is a power failure,
initiating start of the generator,
sensing when the generator voltage and frequency are acceptable,
transferring the load to the alternate source,
monitoring when the Normal source voltage has returned and stabilized for "X" minutes,
transferring the load back to Normal, doing an in-phase transfer if necessary
running the generator unloaded for a cool down period,
shutting down the generator,
initiating a test of the system,
and detecting and alarming problems.

The above is all done automatically in a single package with a proven, tested design. The design of the ATS prohibits the generator from ever being in parallel with the normal source.

The mechanical interlock on distribution board breakers can do the same thing if a PLC or other logic system is added to the breaker controls. Or it can be a cheap, manual operation where the operator manually notices the power outage, starts the generator and closes the breakers. One advantage of the breaker approach is it may be easier to do maintenance on the transfer scheme, if the breakers are draw out style.

A disadvantage of the build your own breaker based transfer system is all the problems that have to be considered and solved. For example, what power supply will the transfer system use and will it be available during a power outage? How do we test the system? What alarms are needed?

I've designed and maintained both ATS and breaker interlocks. The circuit breaker approach is more flexible for special features like on-line testing of the generator or sequential loading, but there is some risk involved. The ATS is basic, automatic and it works.

 

[dpc]

Another consideration is that a typical ASCO type ATS does not provide any overcurrent protection. A circuit breaker scheme does.

 

[waross]

Cost is another factor. In the sizes that I have installed and serviced, I have found that the cheapest solution is an Automatic Transfer Switch, ordered and purchased with the generator.
When I needed a transfer switch "off the shelf" it was much cheaper to "roll my own".
Some caveats:
Motor starting was handled by the gen-set control panel. The ATS need only provide a contact closure.
The gen-sets had their own breaker protection.
The incoming service has a main breaker.
Synchronized transfer is a must with a very fast transfer switch. With breakers or contactors the transfer time is often too long for a synchronized transfer to be effective. It is often better to provide a short time delay to allow the residual back EMF of the induction motors to decay to a safe level.
An off the shelf ATS was generally the most expensive option with the longest delivery time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[masalah]

Deep thanks to all of you colleagues
Really, great supporters