I recently had a customer who lost several computers and other electronic devices to over voltage that resulted from accidental contact between the neighborhood distribution line and a higher voltage feeder. As the neighborhood line went high so did the output of all of the transformers it fed. I'm still in the process of assessing the damage but I wanted some input from the engineering folks on whether grounding both leads of a computer or television power supply when you turn it off could do any harm that I have not anticipated. What I'm thinking about doing is to rewire the power strips that supply those things in this customers home so that when you turn the power strip off you ground all of it's conductors. It would be easy to do because they are controlled by a double pole double throw switch on which only one pole and one throw is used. By wiring the neutral to the center terminal of the other pole of the switch and connecting both of the unused contacts to ground the other end of the switch would ground the power supply leads of anything that is plugged into them whenever it is turned off. I have never worked with modern solid state power supplies. does anyone know of any likely harm that would occur to the power supply circuitry when they are disconnected and grounded very nearly simultaneously.
I'm not to sure what you ment by this: "By wiring the neutral to the center terminal of the other pole of the switch and connecting both of the unused contacts to ground the other end of the switch would ground the power supply leads of anything that is plugged into them whenever it is turned off." I had this same problem but only with Robots.... drives and CPU's burnt out and it causd shit loads of hassle and cost so much to repair. Some ass was welding wilst they were all live. If I were you, I would install a isolator so that when it is switched, It breaks all phase's and Neutral eg single phase 230v & N (I'm from Ireland) so that when switched, it isolates all... and the only thing that's connected is earth (which shouldn't be disconnected!!). Also... it would be an idea to install surge protection (which prob would have prevented the surge). See:
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Obviously the above link is for single phase equip.
You're talking about grounding and shorting all phases when dissconnected from mains... I don't see what this will acomplish... it might damage the PSU's in the equipment... esp the Capacitiors.
It is probable that butching up power strips or other conncected equipment will create a greater hazard/personal liability, praticularly if it's others' gear.
Other-than-AC-power-cable connections to PCs and peripheral and resultant potential difference probably introduces the biggest (overvoltage) destroyer/incinerator/deflagrator of PC-type stuff.
The isolation capability of a mains-rated on/off switch is more than adequate to deal with any over-voltage on the supply line - other than possibly from transients such as caused by lighning strikes. Shorting the equipment's supply wires when switched off is not going to provide any additional protection.
Whilst modern solid state supplies mostly have all their stored energy the "other side" of rectifiers - so that the energy cannot easily find its way back to the input terminals - other equipment may not be so forgiving and your idea is going to increase the risk of damage in general. If you did short out something that had a lot of energy stored, say magnetic energy, shorting it out could do a lot of damage as the designer may not have allowed for this.
The best thing to have if you are worried about irregular supplies is a UPS and a good transient arrest system close to the where the supply enters the property - with a really, really good earth connection. Both are relatively inexpensive, compared to the cost of the equipment they are usually protecting..
I'm not sure just what you hope to accomplish by shorting the power supply ac inputs together. better choice would be to open both the hot and neutral line to the computer. however, it is still possible lightning could find its way to your computer on the ground pin (but not especially likely).
best bet if it worries you is to unplug the power strip. but be aware that shutting off the power to your computer everyday probably does more damage than any transients on the whole.
the chance of the power distribution system doing this again is pretty remote. I would not take any action at all. if you feel you must do something, have a qualified electrician install a whole house suppressor at the service point. while it seems simple, its not a trivial thing to do. many electricians may not understand doing it right.
What it might accomplish is to keep all three connections to the equipment at ground potential. Without any difference of potential across the equipment I had hoped it might avoid damage. The surge protectors that were already installed were destroyed by the over voltage condition which lasted for several minutes rather than the nanoseconds that most surges exist. The switch in the surge protector already opens the hot but the difference between the neutral and the ground were enough to cause damage.
Interesting. My protectors "fail safe" in that they will trip the ground fault breaker (different names in the UK to the Americas) or even blow the main fuse if necessary to protect the house from a sutained over-voltage. I hadn't realised that the US ones were different. I can't much see the point of protectors that allow themselves to be destroyed by going open-circuit in the process. Your neutral
- earth bonding arrangements are clearly different too. Particularly in modern installations, our neutral is well-bonded to the local earth connection so that significant voltage differences cannot exist.
Are you sure the protection circuitry was wired/installed correctly? A bit of a rude question, for which I apologise, but it does seem crazy that they do not fail safe.
Check with the utility they may be paying for the damage.
Grounding of the neutral any other place than the incoming service is a violation of the NEC.
It would be easy to do because they are controlled
Surge protection can help. There are 3 zones of protection for surge devices. Utility, service and point of use. You need 2 of the 3 to be effective. I have a whole house surge protection device on my home and surge protectors at each device that I want to protect. Coordination of the let through voltages will protect most electronic equipment. There is always an active UPS system to protect the investment.
I wouldn't be grounding the wiring systems neutral. I would only be grounding all of the conductors into the power supplies of the devices that are plugged into the surge protector power tap. That would not create any connection between the Grounding Electrode Conductor and the grounded current carrying conductor (neutral).
Important numbers were not provided which makes any accurate response difficult. How much voltage in the transient? For how long? Was is differential mode or common mode? How frequent are these transients - once every hour, day, or year? All questions are for different solutions in different locations.
If plug-in protectors, then they are more often so grossly undersized as to be damaged. The fact that a surge protector is damaged says it was grossly undersized - and therefore provided no effective protection during the first transient.
Numbers are required to better answer your questi> What it might accomplish is to keep all three connections to the
as far as i know; doing this is like shunting a capacitor to ground, you wil only discharge any inline caps and no damage should occur., considering you have true ground and not some other source that may produce a voltage on your grounded side when switching the equipment over.
something like shunting the plug with a screwdriver, needless to say, since lots of electronic circuitry is involved, some caps may still retain their charge but should eventually trickle off harmlessly. but why? Just isolate them OFF from everything and be done with it.
NOTE* I'd probably just transfer the neutral side of the equipment with the switch to grnd to protect it from static & leave the hot side dead & isolated open.
ever wonder why they use shunt proof boots on the plugs.
There is no way I'm going to know the characteristics of the next surge that will hit the premise. What I'm asking is can the connection of the power supply leads of equipment supplied by a surge protector power strip; both neutral and hot; to the equipment grounding conductor of the branch circuit from which they take supply damage the power supply.
I have never found a plug in surge protector that is rated to more than
2000 joules. Is such an animal available?
The plug in units that were damaged were UL listed units rated 1000 joules, 330 volt clamping. The over voltage condition lasted minutes rather than nanoseconds. The re closers on the power line breakers operated the usual three times before they remained open. Since the
7.9K volt line was crossed by a 110 K volt line the over voltage on the output side of the pole pigs may have approached 1670 volts to ground.
Roy Thank you for making an attempt to answer the original question.
If I transfer just the neutral conductor of the power supply to ground and leave the hot lead isolated will the potential difference between them cause the damage I'm trying to avoid?
I'm inclined to transfer both or neither. In either case I'm planning to use the unused pole of the double pole switch to switch the neutral with the hot either from the supply cord conductors to ground or from the supply cord conductors to open.
I would just totally isolate the equipment from anything (circuit/outlet) leading back to the electrical system of your home or office.,
The idea of grounding it electrically when not in use seems sophisticated but unnecesary.those capacitors could keep certain start up circuits or memory chips active, shunting them too often with that type of switching might shorten the computers life.
since you have the option to reconfigure the wires at the switch for the off position, given the nature of your motives and the history of spikes or electrical mishaps in your vicinity, you should just leave your equipment totaly isolated from the circuit, the equipment ground from the box also.
leaving the equipment plugged in but shut off by the switch (both poles) will still leave you with the equipment ground or third prong safety, which is all you need., it's not likely lightening or anything will find it's way to harm your computers through there., so don't shunt to open leads of the switch to anything.,
If you like wire in a different ciruit to those leads with a plot light or led indicating on/off power condition at the receptacle.
Unfortunately you must assume, in advance, what the next transient will be, and then install protection for each of those different transients. A primary to secondary short was suggested. These take seconds and can appear in equipment as either differential or common mode transients. As others have mentioned, these must be shunted at the mains service panel.
To be equivalent to a 3000 joules plug in protector, the 'whole house' protector is only 1000 joules. This is minimal protection. For your type of event, the protector should be
2000 joules or larger. Equivalent plug in protector might be
6000 joules. Square D (among others) makes these larger joules 'whole house' protectors.
Another interesting type more appropriate for your solution is Siemens QSA2020. If transient is so large as to even overwhelm the protector, then the circuit breaker also trips. Circuit breaker cannot be reset if transient has overwhelmed internal protection. This is 'whole house' protector and circuit breaker inside same package. One circuit breaker on each 'critical' circuit would better protect that circuit as well as protect all other circuits serviced by that breaker box.
How to get more than 2000 joules? Install many of these Siemens QSA2020 circuit breakers.
I also designed additional protection in small (and explosive) equipment using a Transzorb and fuse. Transzorb for 120 VAC line was 1.5KE220CA placed across the hot to neutral wire. Transzorbs are not high energy devices but will respond rather quickly to voltages just above minimum. A 2 amp fast blow fuse was small enough so that Transzorb was not explosive. In an event where 180 volts appeared for too many seconds, the Transzorb shorted out the line long enough for the so slow 'fast blow' fuse to eventually open. Of course the Transzorb was destroyed by the event - became a permanent short circuit - as energy numbers from manufacturer data sheet demonstrate. But it shorted long enough so that the fuse eventually blew AND so that critical equipment in a potentially explosive environment would never see excessive voltages.
Also possible to put enough Transzorbs in parallel so that they would not be damaged when blowing the fuse. Transzorbs have a maximum energy rating. But I would not use them on fuses much above two amps.
Shorting the appliance power lead really does nothing better than a power switch. Damage occurs because the transient had both an incoming and outgoing path through equipment. Lets assume the power switch disconnected both hot and neutral wire as so many do. So how did transient get into equipment to cause damage? It can easily enter on safety ground wire. A common mode transient that is more often destructive can enter on any or all three AC power wires. Common mode transients can be incoming even via the safety ground wire. Outgoing on other signaling wires. With black and white (or brown and black) power wires both disconnected or shorted, still, the transient could find destructive circuit paths through the appliance.
Now some power switches disconnect both neutral and hot wire. Others only disconnect the hot wire. Which is on your equipment?
Quash that transient at the service entrance. Again, first ask what the destructive transient will be. Then install protection from each type of possible transient. The most effective first line of defense is the largest joule protector where AC power enters a building.
As demonstrated by example, plug-in protectors don't really claim to provide protection. Therefore they are often undersized - since profits rather than protection is their objective.
In a sales brochure where same primary to secondary line shorted in a Ft Lauderdale home during the Andrew hurricane, Intermatic had a picture of their protector. It was well blackened because the primary wire had fallen on the incoming secondary wire. That protector shunted those high voltages long enough for the primary voltage fuses to eventually open. Protector clearly was destroyed. But nothing - absolutely nothing - inside the home was damaged. This was but a 1000 joule 'whole house' protector.
I think that would still shunt the line circuit of the power supply. I am not sure what effect that would have on the computers circuitry. perhaps none, could be leakage from capacitors discharging through the shunt or maybe the diodes would prevent them from discharging every time you shut the circuit off with conductors bunched up that way.
I wouldn't shunt them like that, in an case. It's your equipment do what you feel is safest & best to protect it.
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