nec 210.4b

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dominicp

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Question arose in last weekends class with Mike on 210.4b. I believe that this refers to all multi branch circuits in regards to simultaneously disconnecting all ungrounded conductors at the point where the branch circuit originates. The question arose with fuse protection, I believe that code states that it must disconnect all if wired in this manner. What is the disconneting means? is it a human being or a fault, automatic means such as load sheding or all the above? how would fuse protection satisfy this requirment? i dont believe it does with a standard disconnect. tks Dominic Pipia
 
I don't think fuses alone would satisfy the requirement. However, a fused disconnect switch would answer the need, so long as the act of throwing the switch will simultaneously turn off all phases of the circuit. The intent is that when you turn off one of the ungrounded conductors that comprises the multi-wire branch circuit, all of the ungrounded conductors associated with that MWBC get turned off. It is a safety issue related to maintenance, not an issue related to the circuit's response to an overload or fault.
 
On a Fusable disconnect, wouldn't the act of a fault on one phase be cause for the MWBC to disconnect only one Phase and not be incompliance with 210.4b
 
In 240.4b it says "a means" is this intrepretation mean it only needs one disconnecting means to satisfy the code? and the handle of the fusable disconnect would satisfy this section?
 
There is a difference between "simultaneous disconnect" (required in 210.4 for all MWBC) and "opened simultaneously" required in 210-4 C Exception 2 for some MWBC's. Simultaneous disconnect means that when someone or something purposely disconnects the circuit (such as for maintenance/repair) the means of disconnection will disconnect all ungrounded conductors. A fused safety switch would do this. "Opened simultaneously" means that when the circuit is opened automatically for a ground fault/short circuit or overload the device would automatically disconnect all ungrounded conductors. A multi-pole breaker would be required for this to occur. Sometime the words get in the way!
 
dominicp said:
On a Fusable disconnect, wouldn't the act of a fault on one phase be cause for the MWBC to disconnect only one Phase and not be incompliance with 210.4b
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Not to repeat myself, but, well OK I'll repeat myself:
charlie b said:
It is a safety issue related to maintenance, not an issue related to the circuit's response to an overload or fault.
Click to expand...
 
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