Isolator or protection.

[STONE85]

Good afternoon,
I know this may sound like a very basic question. Say you have a transformer serving a main mccb panel, why can't the main switch of the panel be just an isolator? I've only seen the main switching device as a mccb or ACB on drawings. Would the protection device on the cable from the transformer not protect the downstream panel?

Thanks

 

[waross]

An isolater may not have an adequate fault interrupting capacity.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[STONE85]

Thanks for this. I assume the same would apply if it was a main switchboard feeding a distribution board or power pillar in that the DB or power pillar would need protection on the incomer ?

Thanks

 

[RRaghunath]

I have seen swtchboards with no incomer protection (only isolator / Disconnector).
But these days, MCCBs don't cost that much extra when compared to Disconnectors and people made it a practice to provide MCCB instead of just diconnector. This is my understanding from what I have seen and discussed with people.

 

[waross]

Under the Canadian Code, in some configurations, transformer fault protection may be provided on the primary side.
In some configurations, overload protection may be provided at either end of a secondary feeder.
In some cases, both safety isolation and transformer overload protection may be provided by one circuit breaker at the panel.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

Not sure of your location, but if you are referring to an overcurrent device on the secondary side of the transformer - in the US: as long as it is sized to protect the bus of your panel or switchboard, no main device at the panel is required at all. Services have special requirements but it's also possible to have a main overcurrent device as the service entrance with the main distribution board located remotely.

 

[che12345]

Dear Mr. STONE85
"... Say you have a transformer serving a main mccb panel, why can't the main switch of the panel be just an isolator? I've only seen the main switching device as a mccb or ACB on drawings. Would the protection device on the cable from the transformer not protect the downstream panel?...".
1. If (your) transformer, i) having [protection on the primary and the secondary], i.e. the length of secondary conductors feeding to the MCCB panel is over-current protected. ii) Attention: in many installations, the primary is fully protected but there is NO protection on the secondary; i.e. the secondary conductors are directly connected to the incoming terminals of the MCCB panel.
2. In case i), the MCCB panel may be provided with isolator or MCCB or ACB for [isolation] purposes. Note: a load isolator is rated to handle the short-circuit current in closed position. It is NOT intended to open or to close on load or short-circuit.
3. In case ii), a MCCB or ACB shall be provided to partially protection of the (unprotected secondary conductors and mainly any over-current/short-circuit in the MCCB panel. Note: A MCCB or ACB are designed to open or to close on to any over-currents; including short-circuits, safely.
4. Whether a MCCB or ACB is depending on current rating and cost. ACB is usually used for current >= 1000A. Note: ACB has better adjustments with 1s (some with 3s ratings), but most MCCB does NOT have or having very low 1s rating. ACBs are always more expansive.
Che Kuan Yau (Singapore)