Disconnect grounded conductor

[csoc64]

Came across an install where the PV utility AC disconnect disconnected both of the ungrounded conductors as well as the grounded (neutral) conductor. System is single phase 120/240V. I had never seen the grounded conductor as a disconnected conductor and it struck me as odd, although I can't explain why. Is there anything in the code (2014) that prohibits disconnecting the grounded conductor? I haven't been able to find anything. Additionally, if this is allowed, is there any practical reason not to disconnect the grounded conductor?

 

[Dennis Alwon]

Nothing in the nec prohibits disconnecting the grounded conductor and in some cases it is required. Not sure if that is the case in your setup. The grounded conductor cannot be disconnected via a fuse where that conductor can be interrupted without the ungrounded conductors

 

[iwire]

This and 404.2(B) apply

240.22 Grounded Conductor. No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:

(1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.

(2) Where required by 430.36 or 430.37 for motor over-load protection.

404.2 Switch Connections.



(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.

Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all cir-cuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor can-not be disconnected until all the ungrounded conductors of the circuit have been disconnected.

 

[jaggedben]

To summarize, it's not a violation unless the disconnect mentioned happens to be a fused disconnect.

In most split-phase PV systems the neutral does not carry current so it is somehat less of a potential safety issue. It is not standard industry practice to switch the neutral though.

 

[GoldDigger]

To summarize, it's not a violation unless the disconnect mentioned happens to be a fused disconnect.

In most split-phase PV systems the neutral does not carry current so it is somehat less of a potential safety issue. It is not standard industry practice to switch the neutral though.

As a practical matter, you would be best off if the disconnect insures that the hot lines are disconnected slightly ahead of the neutral and that the neutral is reconnected slightly ahead of the hot lines. The idea is that you do not want the time difference in making or breaking caused by normal production tolerances and use conditions to ever line up so that the neutral is disconnected first. Otherwise the result could be a momentary (but still fatal to solid state electronics) open neutral condition that could apply L to L voltage across an L to N load.

 

[jaggedben]

As a practical matter, you would be best off if the disconnect insures that the hot lines are disconnected slightly ahead of the neutral and that the neutral is reconnected slightly ahead of the hot lines. The idea is that you do not want the time difference in making or breaking caused by normal production tolerances and use conditions to ever line up so that the neutral is disconnected first. Otherwise the result could be a momentary (but still fatal to solid state electronics) open neutral condition that could apply L to L voltage across an L to N load.

You're not saying this would be an issue, as a practical matter, for a 240V inverter, are you?

 

[GoldDigger]

You're not saying this would be an issue, as a practical matter, for a 240V inverter, are you?

Not for an inverter, no, especially as the typical 240V inverter's active elements normally handle line to line voltage in the first place, regardless of the state of the neutral.
Where the neutral is used for line balance testing I am confident that the inverter would not be damaged by any excursion of the neutral terminal over the range from full L1 voltage to full L2 voltage.

 

[BillK-AZ]

As another practical matter: How does one insure "that the hot lines are disconnected slightly ahead of the neutral and that the neutral is reconnected slightly ahead of the hot lines"? Even test or measure?

 

[GoldDigger]

As another practical matter: How does one insure "that the hot lines are disconnected slightly ahead of the neutral and that the neutral is reconnected slightly ahead of the hot lines"? Even test or measure?

For transfer switches that also switch the neutral (where the generator is configured as an SDS) they do it by deliberately controlling the contact and armature positioning of the neutral and not poles to make that happen. Not cheap.

 

[iwire]

As another practical matter: How does one insure "that the hot lines are disconnected slightly ahead of the neutral and that the neutral is reconnected slightly ahead of the hot lines"? Even test or measure?

As a practical matter we don't. Don't let the engineers mislead you.

It is in the design of the switch or even plug and receptacle.