After some repairing at my house an electrician made an error and didn't co= nnect a neutral on a circuit breaker that was sourcing some electronic equi= pment I have (router, switch, alarm).
The repair work was done on Friday and on Sunday, when I tried to access th= e 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 h= ave a diff of potential so I don't see how the equipment could be damaged b= ut 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.
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.
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 AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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connect a neutral on a circuit breaker that was sourcing some electronic equipment I have (router, switch, alarm).
network I found out that all the electronic equipment was damaged and not firing up (the PSU I suppose).
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.
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.
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
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