Shunt Release or Under Voltage Release for Electrical Interlocking

If you're that worried about it, fit key interlocks. Two locks, one key.

i agree with TonyS.Two locks and one key is the safest way.

However i know consultants insist on electrical interlocking. These guys work in the airconditioned comfort of their offices, and have probably never seen a flimsy auxiliary contact which can easily weld, or vaporise, rendering the electrical interlock null and void. Sigh. In such a case, i would use a shunt trip and not an undervoltage, since a positive logic of tripping (ST) is better than a negative logic (U/V). Economy may dictate that the U/V be used, but that is a poor deign.

I've used both UV and ST. Mainly because I was told to. I wasn't happy with either.

If this is for automatic change over things get interesting.

Mr. KVSridhar, on your comment that shunt is preferred, however with shunt there are chances that if wire breaks, Circuit Breaker would not get command. Installing a TCS would be costly if it is meant for lower rating circuit breakers say 100A or so.

Regards,

So, i assume you recommend the use of U/V for interlocking, So, what is really the raison d'etre of a shunt trip? Why do people offer it at all? Why not use the U/V for everything that a ST can do?

i am not arguing, just curious.

The problem with UV trips is that if power fails, ALL breakers trip and must me manually re-engaged. So the status of the breaker does not remain intact.

This is why key interlocking is the preferred mechanism.

Another option, unfortunately not universally available across all mfrs and all styles of "circuit breakers", is called a "walking bar" mechanism. It is a mechanical interlock between the breaker operators. Only available for MCCBs though.

I would prefer key interlock which will energize UV coil of the ACB, to enable to close the ACB.

In the case of Shunt trip coil ,there will be practical difficulty, because the shunt trip coil will get energized only after closing, there may be chances of momentary making of contacts. This will be detrimental to the circuit in certain applications.

i would prefer a fully mechanical system like Castell for the key locks.

No need to use some flimsy electrical contact in-between.

ST in an interlock system is powered by an independent source. So, not dependent on the CB status. Most STs nowadays are electronically controlled, and are full-time rated, so no clearing contact from the breaker is wasted on the ST.

Just my opinion. Not infallible.

Agreed.

But in certain applications like auto transfer schemes fully castle lock system can not be used.

Moreover now a days most of the incomer ACBs are electrical operated ACBs with remote closing facility, in such cases only Shunt or UV coils have to be used.

That sounds like a good solution. The Castell key system works well if the environmental conditions are poor or if the standard of operating personnel is questionable.

Which out of 2? Shunt or UVR? Which is more reliable?

In the good old days, the UV was directly across the voltage it was monitoring. A slowly dropping voltage would result in a similar slow dropoff of the UV, and the CB would not trip. However, nowadays, in some CBs, UV , ST and CC are electronically controlled and act in a bistable way, with a definite snap. So, UVs of today by reputed companies are good. But i still feel logically ST should be used for electrical interlocking. UV is used only to save money, and is a compromise.

Castell type key locks are the best of course. It seems that you don't want them.

But Shunt would have a problem that if wire breaks, it would not give function what it is meant for which in case of UVR would be more reliable.

TCS is a costly affair as I am also talking about small rating CBs wherein MCCBs are used for interlocking.

Wire breaking can happen anywhere. Too unlikely.

Some small MCCBs have a 'walking beam' interlock. JRaef mentioned this. Here is an image of a Siemens Walking Beam interlock.