18 years ago
utility power, while having a grounding wire that is still grounded.
Such disconnection would mean disconnecting the neutral wire, and do so
not before disconnecting the live phase wires. As far as I can tell, a
circuit breaker that disconnects all wires at the same time would comply
with NEC code (so for a 240/120 volt single phase feed, using a three
pole breaker normally intended for three phase would get the job done).
The issue is with grounding. It's been pointed out to me that in NEC
250.24(A)(1), the grounded wire is to be connected to the grounding wire
between the serice drop/lateral, and the terminals of the disconnecting
means. Then 250.24(A)(5) prohibits connecting at any points on the load
side of the disconnecting means.
The problem I have with this, is that by connecting the grounded conductor
before the disconnection, it really isn't a 100% disconnection since the
grounding system (e.g. the grounding wire, not the neutral wire) in the
building is still attached to the utility neutral wire. Faults or breaks
in the utility wiring, or even in neighbor wiring sharing the same
transformer, could introduce voltage relative to ground on the neutral
wire. While the grounding connection would make for a path for that
voltage to go to ground, no ground is ever perfect, so if there is ever
any other point in the grounding system (e.g. any equipment which has a
chassis attached to the grounding wire) which might get grounded to earth,
it also forms a path where current could flow. If that path includes a
human being, then what we have is two paths in parallel, one near the
service entrance (the normal grounding point), and one including a human.
Since ground is a resistive component (relative to ground return paths)
this will mean some part of the return current flows through the human.
Since the grounding system has become part of the circuit under a supply
fault, it is not doing its job in protecting people.
A ground fault detector at the main disconnect won't detect this since it
is current flowing on the grounding wire itself once it has reached that
wire from the connection with the neutral. Even if a disconnect breaker
included disconnecting the neutral (allowed by NEC only if the neutral is
disconnected with or after the other conductors), it won't stop this
What I propose is that where a main service disconnect also disconnects
the neutral conductors (simultaneously with all other conductors), that
the neutral be connected to the grounding wire on the load side of the
disconnect. This way, when the conductors are disconnected, there is no
possible path for a supply/line side fault to energize the grounding wires
in the building. This will also allow a ground fault current detection to
work properly since it will be detecting all current coming in that does
not have an equivalent return current. Such a detector would then be able
to interrupt such faults via a shunt trip or such on the main disconnect.
I generally find when the NEC has a rule, there's a good reason for it.
But I cannot figure out the reason for having the grounding point of the
grounded conductor (neutral) being on the line side of the main breaker,
as opposed to the load side. Obviously, in cases (and this is most of them)
where the neutral is not switched at all, it's moot, electrically. But it
is allowed to disconnect the neutral when the other conductors are as well,
and in such a case, it now matters which side things are on. But I just do
not see the reason why the NEC has it this way, and as far as I can see, it
is safer as I proposed.
Reference from the NEC 2005 draft:
* 250.24 Grounding Service-Supplied Alternating-Current Systems.
* (A) System Grounding Connections. A premises wiring system supplied by a
* grounded ac service shall have a grounding electrode conductor connected
* to the grounded service conductor, at each service, in accordance with
* 250.24(A)(1) through (A)(5).
* (1) General. The connection shall be made at any accessible point from
* the load end of the service drop or service lateral to and including the
* terminal or bus to which the grounded service conductor is connected at
* the service disconnecting means.
* (5) Load-Side Grounding Connections. A grounding connection shall not be
* made to any grounded conductor on the load side of the service disconnecting
* means except as otherwise permitted in this article. [ROP 5 65]