Hi, guys,

Question for those knowlegable about electrician and code type stuff..

I have a situation where there we are wiring what is, in effect, a
transfer switch which will connect external 120VAC/60Hz power to two
sets of loads (sort of like the 240VAC/center tapped situation in a
home) OR a pair of 2kW inverters.

When external power is supplied by a line cord, the neutral and ground
are effectively tied together, however once the line cord is
disconnected the ground (chassis etc.) and the inverter neutrals are
tied to each other, but floating with respect to the ground.

Are there safety or "code" (not sure any code really applies to this
situation when it's unplugged and far away from any power lines)
issues with letting neutral and ground float wrt each other in an
inverter power situation from an electrical wiring point of view?



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

The code is simple. Neutral is tied to safety earth at only one location or
ground loops are formed and by convention this is at the source of power.
Another rule is that safety earth should never be switched. So, a source of
power is a distribution transformer or a generator (read inverter) All other
electrical connections are consumers. Consumers use distributer connections
for phase, neutral and safety earth. Consumers are not allowed to create
their own earth.

In the case of multiple sources and a common load, there can only be one
earth reference and neutral of each source is connected to that singular
earth reference at each energy source. Ideally, all sources should be as
close as possible to the earth reference.
Steve


"Spehro Pefhany" wrote in message
...
Hi, guys,

Question for those knowlegable about electrician and code type stuff..

I have a situation where there we are wiring what is, in effect, a
transfer switch which will connect external 120VAC/60Hz power to two
sets of loads (sort of like the 240VAC/center tapped situation in a
home) OR a pair of 2kW inverters.

When external power is supplied by a line cord, the neutral and ground
are effectively tied together, however once the line cord is
disconnected the ground (chassis etc.) and the inverter neutrals are
tied to each other, but floating with respect to the ground.

Are there safety or "code" (not sure any code really applies to this
situation when it's unplugged and far away from any power lines)
issues with letting neutral and ground float wrt each other in an
inverter power situation from an electrical wiring point of view?



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com

On Tue, 8 Jan 2008 00:10:18 +0100, the renowned "Steve Lusardi"
wrote:

The code is simple. Neutral is tied to safety earth at only one location or
ground loops are formed and by convention this is at the source of power.
Another rule is that safety earth should never be switched. So, a source of
power is a distribution transformer or a generator (read inverter) All other
electrical connections are consumers. Consumers use distributer connections
for phase, neutral and safety earth. Consumers are not allowed to create
their own earth.

In the case of multiple sources and a common load, there can only be one
earth reference and neutral of each source is connected to that singular
earth reference at each energy source. Ideally, all sources should be as
close as possible to the earth reference.
Steve

So, it would seem to follow inescapably that the neutrals must be
switched where there are two alternative sources of power, even if
they are isolated from each other, yes?

On Mon, 07 Jan 2008 16:10:27 -0500, Spehro Pefhany
wrote:

Hi, guys,

Question for those knowlegable about electrician and code type stuff..

I have a situation where there we are wiring what is, in effect, a
transfer switch which will connect external 120VAC/60Hz power to two
sets of loads (sort of like the 240VAC/center tapped situation in a
home) OR a pair of 2kW inverters.

When external power is supplied by a line cord, the neutral and ground
are effectively tied together, however once the line cord is
disconnected the ground (chassis etc.) and the inverter neutrals are
tied to each other, but floating with respect to the ground.

Are there safety or "code" (not sure any code really applies to this
situation when it's unplugged and far away from any power lines)
issues with letting neutral and ground float wrt each other in an
inverter power situation from an electrical wiring point of view?

Safety concern: Almost sounds like you are running two separate
120V inverters in series to get 120/240V 1Ph center-tapped power.

I sincerely doubt the inverters are going to like that arrangement
at ALL, since there isn't any mechanism to lock them in frequency sync
with each other while off grid, or to handle parallel load sharing.

It might work if all the loads are 120V connected between the
separate hot lines and the grounded line. For switching over two
separate sets of 120V equipment that are wired on a "three-wire
circuit" for ease of cabling. And both sides of an average inverter
are floating in regards to safety ground - unless they have their
Neutral tied to the internal chassis ground, and you wire the chassis
ground to an earth ground reference point.

But any 240V connected loads phase-to-phase are going to see a wild
ride on the voltage as the inverters heterodyne against each other, at
least till the "Magic Smoke" from one or both inverters puts an end to
the experiment.

The power source needs to be designed for the load being driven.

-- Bruce --

On Mon, 07 Jan 2008 21:40:05 -0800, the renowned Bruce L. Bergman
wrote:

On Mon, 07 Jan 2008 16:10:27 -0500, Spehro Pefhany
wrote:

Hi, guys,

Question for those knowlegable about electrician and code type stuff..

I have a situation where there we are wiring what is, in effect, a
transfer switch which will connect external 120VAC/60Hz power to two
sets of loads (sort of like the 240VAC/center tapped situation in a
home) OR a pair of 2kW inverters.

When external power is supplied by a line cord, the neutral and ground
are effectively tied together, however once the line cord is
disconnected the ground (chassis etc.) and the inverter neutrals are
tied to each other, but floating with respect to the ground.

Are there safety or "code" (not sure any code really applies to this
situation when it's unplugged and far away from any power lines)
issues with letting neutral and ground float wrt each other in an
inverter power situation from an electrical wiring point of view?

Hi, Bruce, thanks for your comments--

Safety concern: Almost sounds like you are running two separate
120V inverters in series to get 120/240V 1Ph center-tapped power.

Nah, there are two inverters because one will not handle the multi-kW
loads (there are a number of loads on each inverter). The inverters
are not sychronized so the AC voltage between the two "lines" might be
anyhere from zero to 2 * 120VAC at any given time. It's a bit of
complexity that's not completely germane to the issue-- I mention it
ONLY because it affects the total voltages that might show up, and
thus safety. The neutrals are tied together because the intention was
to switch (transfer) only the "hot" lines, but I'm thinking now that's
not a good idea.

I sincerely doubt the inverters are going to like that arrangement
at ALL, since there isn't any mechanism to lock them in frequency sync
with each other while off grid, or to handle parallel load sharing.

There are four similar parallel loads on each. No current flows
between the neutrals when operating on inverter power.

It might work if all the loads are 120V connected between the
separate hot lines and the grounded line. For switching over two
separate sets of 120V equipment that are wired on a "three-wire
circuit" for ease of cabling. And both sides of an average inverter
are floating in regards to safety ground - unless they have their
Neutral tied to the internal chassis ground, and you wire the chassis
ground to an earth ground reference point.

Correct. But if we tie the neutral to the chassis at the inverter then
we must switch the neutral from the inverter AND from the external
power, right? (transfer the connection) Otherwise we have two
'grounds' when external power is supplied.

But any 240V connected loads phase-to-phase are going to see a wild
ride on the voltage as the inverters heterodyne against each other, at
least till the "Magic Smoke" from one or both inverters puts an end to
the experiment.

Sure, it's not a problem in this case.

The power source needs to be designed for the load being driven.

-- Bruce --

On Mon, 07 Jan 2008 21:40:05 -0800, Bruce L. Bergman
wrote:


Almost sounds like you are running two separate
120V inverters in series to get 120/240V 1Ph center-tapped power.

I sincerely doubt the inverters are going to like that arrangement
at ALL, since there isn't any mechanism to lock them in frequency sync
with each other while off grid, or to handle parallel load sharing.

You might enjoy reading the manual for Trace SW series inverters.
http://www.xantrex.com/web/id/307/docserve.asp Pg 43, A-3, and B-1. I
have a pair of 4024s connected to a conventional panel. Mixed 120V and
240V loads.

Wayne

Yes, absolutely. With single phase loads, neutral is active and I must point
out again, do not connect neutral to the inverter chassis. The inverter
internally will connect safety earth to the inverter chassis. Do not tie
multple inverters together unless they can be synchronized and they must be
designed to do that. If you intend to power a common load with two
inverters, they cannot do this together. It is one or the other.
Steve

"Spehro Pefhany" wrote in message
...
On Tue, 8 Jan 2008 00:10:18 +0100, the renowned "Steve Lusardi"
wrote:

The code is simple. Neutral is tied to safety earth at only one location
or
ground loops are formed and by convention this is at the source of power.
Another rule is that safety earth should never be switched. So, a source
of
power is a distribution transformer or a generator (read inverter) All
other
electrical connections are consumers. Consumers use distributer
connections
for phase, neutral and safety earth. Consumers are not allowed to create
their own earth.

In the case of multiple sources and a common load, there can only be one
earth reference and neutral of each source is connected to that singular
earth reference at each energy source. Ideally, all sources should be as
close as possible to the earth reference.
Steve

So, it would seem to follow inescapably that the neutrals must be
switched where there are two alternative sources of power, even if
they are isolated from each other, yes?

On Tue, 8 Jan 2008 18:30:33 +0100, "Steve Lusardi"
wrote:

Yes, absolutely. With single phase loads, neutral is active and I must point
out again, do not connect neutral to the inverter chassis. The inverter
internally will connect safety earth to the inverter chassis. Do not tie
multple inverters together unless they can be synchronized and they must be
designed to do that. If you intend to power a common load with two
inverters, they cannot do this together. It is one or the other.
Steve

Why not tie neutral to the inverter chassis ? That's effectively the
same thing as having ground at the source. In this case a literal
ground will be located (hopefully) well away from the inverters, so
chassis is as close as we got.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Tue, 08 Jan 2008 14:56:39 -0500, Spehro Pefhany
wrote:

On Tue, 8 Jan 2008 18:30:33 +0100, "Steve Lusardi"
wrote:

Yes, absolutely. With single phase loads, neutral is active and I must point
out again, do not connect neutral to the inverter chassis. The inverter
internally will connect safety earth to the inverter chassis. Do not tie
multple inverters together unless they can be synchronized and they must be
designed to do that. If you intend to power a common load with two
inverters, they cannot do this together. It is one or the other.
Steve

Why not tie neutral to the inverter chassis ? That's effectively the
same thing as having ground at the source. In this case a literal
ground will be located (hopefully) well away from the inverters, so
chassis is as close as we got.

The relevant parts of the NEC are those that deal with "Separately
Derived Systems." One common example of a separately derived system is
a control system with a 480V/120V transformer that powers sensors,
pilot devices, contactors, etc. Usual practice is to bond one side of
the secondary of the control xformer to the equipment frame. That
conductor becomes a neutral.

I don't have a clear enough picture of your equipment in all its
possible configurations to tell whether the NEC would consider it a
separately derived system. Is it portable? Is there an earth ground
other than thru the line cord?

--
Ned Simmons

On Tue, 08 Jan 2008 16:08:56 -0500, Ned Simmons
wrote:

On Tue, 08 Jan 2008 14:56:39 -0500, Spehro Pefhany
wrote:

On Tue, 8 Jan 2008 18:30:33 +0100, "Steve Lusardi"
wrote:

Yes, absolutely. With single phase loads, neutral is active and I must point
out again, do not connect neutral to the inverter chassis. The inverter
internally will connect safety earth to the inverter chassis. Do not tie
multple inverters together unless they can be synchronized and they must be
designed to do that. If you intend to power a common load with two
inverters, they cannot do this together. It is one or the other.
Steve

Why not tie neutral to the inverter chassis ? That's effectively the
same thing as having ground at the source. In this case a literal
ground will be located (hopefully) well away from the inverters, so
chassis is as close as we got.

The relevant parts of the NEC are those that deal with "Separately
Derived Systems." One common example of a separately derived system is
a control system with a 480V/120V transformer that powers sensors,
pilot devices, contactors, etc. Usual practice is to bond one side of
the secondary of the control xformer to the equipment frame. That
conductor becomes a neutral.

I don't have a clear enough picture of your equipment in all its
possible configurations to tell whether the NEC would consider it a
separately derived system. Is it portable? Is there an earth ground
other than thru the line cord?

Not really. It's in a moving vehicle.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Tue, 08 Jan 2008 17:04:05 -0500, Spehro Pefhany
wrote:

On Tue, 08 Jan 2008 16:08:56 -0500, Ned Simmons
wrote:


I don't have a clear enough picture of your equipment in all its
possible configurations to tell whether the NEC would consider it a
separately derived system. Is it portable? Is there an earth ground
other than thru the line cord?

Not really. It's in a moving vehicle.

The NEC does have a section on RVs, which even if not directly
applicable, may give some guidance.

Based on my understanding of the situation, I'd create a neutral by
bonding the common connection between the inverters to the vehicle's
chassis, and switch the neutral along with the hots when selecting
inverter or utility power.

I might reconsider if the inverter batteries can be charged by utility
power, and there isn't isolation somewhere between the utility input
to the charger and the inverters' outputs.

--
Ned Simmons

On Wed, 09 Jan 2008 00:03:42 -0500, Ned Simmons
wrote:

On Tue, 08 Jan 2008 17:04:05 -0500, Spehro Pefhany
wrote:

On Tue, 08 Jan 2008 16:08:56 -0500, Ned Simmons
wrote:


I don't have a clear enough picture of your equipment in all its
possible configurations to tell whether the NEC would consider it a
separately derived system. Is it portable? Is there an earth ground
other than thru the line cord?

Not really. It's in a moving vehicle.

The NEC does have a section on RVs, which even if not directly
applicable, may give some guidance.

Based on my understanding of the situation, I'd create a neutral by
bonding the common connection between the inverters to the vehicle's
chassis, and switch the neutral along with the hots when selecting
inverter or utility power.

I might reconsider if the inverter batteries can be charged by utility
power, and there isn't isolation somewhere between the utility input
to the charger and the inverters' outputs.

Inverters are running from a 300A generator on the vehicle power
system, and they provide input-output isolation.

Thanks, Ned, and everyone else who offered suggestions.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com