Neutral fault damaged several PSU - Is it possible ?

[JC]

Hi !

After some repairing at my house an electrician made an error and didn't connect a neutral on a circuit breaker that was sourcing some electronic equipment I have (router, switch, alarm).

The repair work was done on Friday and on Sunday, when I tried to access the network I found out that all the electronic equipment was damaged and not firing up (the PSU I suppose).

At first sight, it seems that if the neutral is not connected we wouldn't have a diff of potential so I don't see how the equipment could be damaged but live & earth were ok so I wonder if anyone can explain to me if the lack of neutral could have been the cause.

Thanks/Brgds
J

 

[Jon Elson]

JC wrote:

Hi !

After some repairing at my house an electrician made an error and didn't
connect a neutral on a circuit breaker that was sourcing some electronic
equipment I have (router, switch, alarm).

The repair work was done on Friday and on Sunday, when I tried to access
the network I found out that all the electronic equipment was damaged and
not firing up (the PSU I suppose).

At first sight, it seems that if the neutral is not connected we wouldn't
have a diff of potential so I don't see how the equipment could be damaged
but live & earth were ok so I wonder if anyone can explain to me if the
lack of neutral could have been the cause.
This is in the US or other country that has most appliances tied between hot

and neutral? Then, an open neutral can cause those appliances to have
anywhere netween 0 and 240 V on them, depending on the combination of
appliances in series on each side of the isolated neutral. If you turn on
a toaster, microwave oven or other heavy load on one side, and have a light
load on the other side, then you will get much above normal line voltage on
the lightly loaded side. Many more modern pieces of computer gear will have
wide-range (86 - 265V) supplies, and shouldn't be damaged by this sort of
incident.

If you are in a country where the mains are 240 V unbalanced (with a
neutral) then the same sort of problem can exist, but the voltages could
be higher, enough to damage even the wide-range devices.

Jon

 

[Dave Platt]

In article <KcKdnTv25ZGuVMXNnZ2dnUVZ_sidnZ2d@giganews.com>,
Jon Elson <jmelson@wustl.edu> wrote:

At first sight, it seems that if the neutral is not connected we wouldn't
have a diff of potential so I don't see how the equipment could be damaged
but live & earth were ok so I wonder if anyone can explain to me if the
lack of neutral could have been the cause.

This is in the US or other country that has most appliances tied between hot
and neutral? Then, an open neutral can cause those appliances to have
anywhere netween 0 and 240 V on them, depending on the combination of
appliances in series on each side of the isolated neutral. If you turn on
a toaster, microwave oven or other heavy load on one side, and have a light
load on the other side, then you will get much above normal line voltage on
the lightly loaded side.

Yup. Here in the U.S., if you turn on a heavy load, and other lights
in the area get brighter (rather than dimmer) or other motors speed up
(rather than slow down), you've probably got a loose or open neutral.

The reason it works these way, is that most houses in the U.S. have a
three-wire drop from the pole pig (transformer) - the transformer's
secondary winding is center-tapped, and you get wires from both ends
of the winding and from the center tap. The wire from the center tap
is what becomes your "neutral", and it's connected to your building
ground point (typically right at the power distribution panel).

The two different "end of transformer winding" wires are each at 120
volts potential with respect to the center tap (neutral), but are 180
degrees out of phase with one another. Each wire feeds some fraction
of the 120-volt branch circuits in your house. There are often some
240-volt outlets as well - in these, each of the two "hot" terminals
is fed from one of the two opposite-phase 120-volt wires, and so
there's 240 volts between the two.

If you're drawing exactly equal amounts of current from the two
120-volt feeds, then all of the current is flowing through these
wires, and none flows back through neutral to the transformer center
tap. That's rarely the case, though, and you're usually drawing
different amounts of current through the two circuits... in which
case, the current difference flows back through the neutral wire.

If the neutral wire opens, though, this latter situation *cannot*
occur... no current can flow through neutral to the transformer. The
entire transformer secondary winding, and your house's two sets of
branch circuits, are all a single circuit in series... both 120-volt
drop lines, and both sides of the house circuit are *forced* to carry
identical currents.

This means that if you add an additional load on one side of the house
wiring (e.g. turn on a motor), this will increase the current flow in
*both* sides of the house wiring... and the only way this can happen
is for "neutral" to be pulled several volts in the direction of the
load you've just added, increasing the voltage across the opposite
side of the house wiring so that everything on that side is now
drawing more current.

Worst case - a big heavy load trying to operate on one side, and one
light load on the other. The heavy load ends up with very little
voltage, and the light load sees over 200 volts. PFFT.

This sort of thing has happened at my house a couple of times - in
each case, a squirrel had chewed through the neutral wire in the
pole-pig-to-house drop cable.

When I called our power company, they said that this sort of "low
voltage, high voltage" situation is treated as very urgent - they sent
out a repair team immediately. The risk of equipment damage, and
possibly an electrical fire is significant.

Fortunately, my house suffered no damage other than a couple of
burned-out incandescent light bulbs.

Many more modern pieces of computer gear will have
wide-range (86 - 265V) supplies, and shouldn't be damaged by this sort of
incident.

It's probably older computer gear, consumer-electronic devices with
linear power supplies, motors, and lights that are most at risk from
the resulting over- and under-voltage conditions.

Also, if you've got other "ground" connections (e.g. a cable TV feed),
and these are not properly bonded to your own building's ground, I
believe that an "open neutral" could result in some significant
current flows going through the various "ground" connections, and this
might cause damage as well.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

On Tue, 18 Sep 2012 11:45:43 -0700 (PDT), JC <jcasqueiro@netcabo.pt>
wrote:

Hi !

After some repairing at my house an electrician made an error and didn't connect a neutral on a circuit breaker that was sourcing some electronic equipment I have (router, switch, alarm).

The repair work was done on Friday and on Sunday, when I tried to access the network I found out that all the electronic equipment was damaged and not firing up (the PSU I suppose).

At first sight, it seems that if the neutral is not connected we wouldn't have a diff of potential so I don't see how the equipment could be damaged but live & earth were ok so I wonder if anyone can explain to me if the lack of neutral could have been the cause.

Thanks/Brgds
J

Saw that happen in a network lab when they added a 40KW UPS and
switched hot and neutral on one side of the split 120/240 circuit
(US). Didn't take long to notice there was a problem - the power
supplies on the Cisco equipment got very noisy but most survived. The
PC's immediately gave up their magic smoke. I don't know what the
final replacement cost was, but can guess it was $$$$$.

The foreman of the installation crew said that the uncaught error by
one his electricians would probably cost him (the foreman) his job.

 

[Phil Allison]

"JC"

After some repairing at my house an electrician made an error and didn't
connect a neutral on a circuit breaker that was sourcing some electronic
equipment I have (router, switch, alarm).

The repair work was done on Friday and on Sunday, when I tried to access the
network I found out that all the electronic equipment was damaged and not
firing up (the PSU I suppose).

At first sight, it seems that if the neutral is not connected we wouldn't
have a diff of potential so I don't see how the equipment could be damaged
but live & earth were ok so I wonder if anyone can explain to me if the lack
of neutral could have been the cause.


** A disconnected neutral would normally result in no AC power on that
circuit as the earth conductor is not used for carrying load current.

Only if the neutral conductor were common to more than one phase can a
damaging over voltage exist. This may sometimes be the case in the USA, but
is not permitted in 240 volt countries for domestic installations.


.... Phil

 

[Uffe Bćrentsen]

Den 19-09-2012 00:12, Phil Allison skrev:

"JC"

After some repairing at my house an electrician made an error and didn't
connect a neutral on a circuit breaker that was sourcing some electronic
equipment I have (router, switch, alarm).

The repair work was done on Friday and on Sunday, when I tried to access the
network I found out that all the electronic equipment was damaged and not
firing up (the PSU I suppose).

At first sight, it seems that if the neutral is not connected we wouldn't
have a diff of potential so I don't see how the equipment could be damaged
but live & earth were ok so I wonder if anyone can explain to me if the lack
of neutral could have been the cause.


** A disconnected neutral would normally result in no AC power on that
circuit as the earth conductor is not used for carrying load current.

Only if the neutral conductor were common to more than one phase can a
damaging over voltage exist. This may sometimes be the case in the USA, but
is not permitted in 240 volt countries for domestic installations.

If the neutral wire in the feeder gets disconnected then you will have
random voltages anywhere in the installation.
Voltage will be anywhere between normal line voltage 115V in US or 230V
here in DK and (sqrt3 * line voltage)
US: sqrt3 * 115 = 200V
DK: sqrt3 * 230 = 400V


--
Uffe

 

[Phil Allison]

"Uffe Bćrentsen"
Phil Allison skrev:

"JC"

After some repairing at my house an electrician made an error and didn't
connect a neutral on a circuit breaker that was sourcing some electronic
equipment I have (router, switch, alarm).

The repair work was done on Friday and on Sunday, when I tried to access
the
network I found out that all the electronic equipment was damaged and not
firing up (the PSU I suppose).

At first sight, it seems that if the neutral is not connected we wouldn't
have a diff of potential so I don't see how the equipment could be
damaged
but live & earth were ok so I wonder if anyone can explain to me if the
lack
of neutral could have been the cause.


** A disconnected neutral would normally result in no AC power on that
circuit as the earth conductor is not used for carrying load current.

Only if the neutral conductor were common to more than one phase can a
damaging over voltage exist. This may sometimes be the case in the USA,
but
is not permitted in 240 volt countries for domestic installations.

If the neutral wire in the feeder gets disconnected ...

** Clearly NOT the OP's question.


then you will have

random voltages anywhere in the installation.
Voltage will be anywhere between normal line voltage 115V in US or 230V
here in DK and (sqrt3 * line voltage)
US: sqrt3 * 115 = 200V
DK: sqrt3 * 230 = 400V

** In the USA, 120V domestic circuits are fed via a split phase transformer
from a 3 phase, 240V street supply.

Most homes have 240V and 120V power available, the 240V being used for high
consumption appliances.

The 240v is really 120V-0-120V - so a loss of the incoming neutral can
case the supply to go to 240V.


..... Phil