?solated Neutral Systems Phase-Neutral MCB

[karalahana]

hi, we use Phase+neutral breaking MCB in isolated neutral systems. I wonder one thing.
For a 120 volt say 6 A circuit is switched by 6 A MCB
so for a 120 volt 6 A circuit ?solated neutral circuit if mcb is rated 6 amper it means it has two contacts of which each has the same capacity of the first MCB cited above , or they have 3 amper capacity 2 contacts?
I wonder how effects the number of contacts in series in way of breaking capacity.is there any general formulation to use say in contactors so that we can use multiple contactors to break high currents with small-rated contactors or mcbs? I want some information of this issue...

 

[waross]

If a breaker is rated at 6 amps, all the poles will be rated for 6 amps amd the tripping current will be based on 6 amps.
Relay poles may be connected in series to handle greater voltages. Some multi pole breakers may have AC and DC voltage ratings and the DC rating may be based on all poles being connected in series.
Your local code will prevail. There may be code issues connecting breaker poles in series to increase the voltage rating in the breaker is not specifically listed for use with the poles in series.
Current ratings:
It is considered bad practice and is contrary to many codes to connect breaker or relay poles in parallel to increase the rating at such small currents. Some codes allow parallel connections under specific conditions for higher currents. Such instances may be for hundreds a or possibly thousands of amps.
At 6 amps, I suggest buying a higher rated breaker.

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

 

[rbulsara]

There are no formula for determining contact ratings. They need to be tested and listed as such, in most countries.

You are making and breaking 6 amps in the phase wire and 6 amps in the neutral, you need at least 6 amps continuous rated and also be able to make/break the available short circuit current, at both points. Rating also have to be suitable for the type of load and current ( power factor, AC or DC).

Rafiq Bulsara

http://www.srengineersct.com