Are "all mode" surge protectors a threat to the human safety?

[Anonymous user]

Hi!

I've been reading some stuff on the safety issues of using "all mode" or "three mode" surge protectors, those which divert surges to safety ground, so I started being concerned about human safety.

Imagine an MOV start conducting current from Phase --> Ground either because the MOV has failed or in the case of a surge being diverted to ground. Wouldn't it be a threat for the safety of those touching exposed metal parts which are attached to the same safety ground?

I don't know how this is in other countries, but in my country, electric showers are very common and in my house there are three of them which are all connected to the same safety ground the computers are plugged to and the safety ground is bonded together with the neutral wire at the entrance of my house.

I've been using APC products for quite a while and never had a problem, but I noticed that all of them divert surges to ground and most of them are "all mode" protectors, even the UPSses. I think it's important to say that in some products theres is nothing to disconnect falining MOVs connected from neutral to ground in case of failure. And also, in case they start conducting for good, after failing, a "new" ground-neutral bond is made inside your house and you're neve gonna know this happend and the current coming from the appliences is now divided, so half of it will follow the neutral path and the other half will follow the safety ground path and now the ground is no longer a safety path.

So, what you guys have to tell me about it? Will I be electrocuted while taking a shower in case of an MOV start conducting between Hot-Ground or Neutral-Ground?? It doesn't even need to be diverting a surge, the MOV just need to fail and and most of us know or should know,they can and will fail without being hit by a surge.

While this is not very clear to me, I've removed all the MOVs connected to ground from my UPSes and Surge protectors. If I fiund out this is not a safety issue, I put them back again.

Thanks!

Message was edited by: rau

Message was edited by: rau

[Anonymous user]

Hi!

I've been reading some stuff on the safety issues of using "all mode" or "three mode" surge protectors, those which divert surges to safety ground, so I started being concerned about human safety.

Imagine an MOV start conducting current from Phase --> Ground either because the MOV has failed or in the case of a surge being diverted to ground. Wouldn't it be a threat for the safety of those touching exposed metal parts which are attached to the same safety ground?

I don't know how this is in other countries, but in my country, electric showers are very common and in my house there are three of them which are all connected to the same safety ground the computers are plugged to and the safety ground is bonded together with the neutral wire at the entrance of my house.

I've been using APC products for quite a while and never had a problem, but I noticed that all of them divert surges to ground and most of them are "all mode" protectors, even the UPSses. I think it's important to say that in some products theres is nothing to disconnect falining MOVs connected from neutral to ground in case of failure. And also, in case they start conducting for good, after failing, a "new" ground-neutral bond is made inside your house and you're neve gonna know this happend and the current coming from the appliences is now divided, so half of it will follow the neutral path and the other half will follow the safety ground path and now the ground is no longer a safety path.

So, what you guys have to tell me about it? Will I be electrocuted while taking a shower in case of an MOV start conducting between Hot-Ground or Neutral-Ground?? It doesn't even need to be diverting a surge, the MOV just need to fail and and most of us know or should know,they can and will fail without being hit by a surge.

While this is not very clear to me, I've removed all the MOVs connected to ground from my UPSes and Surge protectors. If I fiund out this is not a safety issue, I put them back again.

Thanks!

Message was edited by: rau

Message was edited by: rau

[voidstar_apc]

I'd be interested in taking a look at what you're reading.

You can certainly be injured if your electrical wiring is improperly grounded. Without a good ground connection, a surge diverted to ground could go through someone. APC's UPSes have site wiring fault indicators which detect these dangerous conditions. A good ground connection provides a safe path for the energy.

One of your questions is why it is safe for ground to be exposed to human contact. This is because you have a very large resistance and metal has a negligible resistance. Take aluminum. Three meters of aluminum at roughly 2.7*10^-8 ohms per meter is 81 nOhms. A human is about 1 MOhm when dry, but even if a human were 1 Ohm, it would have no effect on the parallel resistance. Electricity really wants to go through the metal, not you.

You also asked what happens if a MOV starts conducting for good? As I learned in my foolish younger days, conductors placed between phase and neutral or phase and ground destroy themselves. And then the circuit breaker trips. In the case of a surge protector the thermal fuse opens to safely stop the short circuit.

Perhaps someone more knowledgeable than me will point out something I'm missing, but I have faith in the electric codes and uL to keep me safe. They've had decades to work out the issues. They require safe practices in building wiring and consumer equipment.

[Anonymous user]

Thanks for your reply!

My concern is not lightning strikes since it's advisable not to take a shower or use the telephone or handle metal stuffs connected to an electrical wire during lightning storms anyway. My concern is really the varistors using the safety ground for diverting excessive energy (and it doesn't need to be from a lightning strike) and also when MOV fails, and they do fail! When they fail they short both wires, as far as I know. Sometimes it destroys itself completely, but there are other times it just shorts out. If the phase wire and ground wire are shorted together, not a problem here, since the circuit breaker will trip anyway, but when MOVs placed between Neutral and Ground, there's nothing made to disconnect it, they will be bonded together until you replace the MOV or the surge protector, but there is no way you can know it's happened.

The things I've been reading says the american code don't allow neutral and ground bonds inside the house since this will create two paths for the returning current coming from appliences to follow and that should never happen since it is a safety issue. Take a look at this document, it says everthing I'm telling you here:

http://www.transtector.com/DataSheets/ViewDocs/1457-002.pdf

On www.surgex.com and www.zerosurge.com there are also some documents that condemn MOVs and the way they use the safety ground, since it can also disrupt interconnected electronics and damage datacom ports. I know they want to sell their products, but what they state is very convincing.

I don't know if this issue has been discussed in my country, but here it's hard to find surge protectors or UPSses that use the safety ground for diverting surges, most of them are Phase to Neutral apart from those made by APC. 90% of the products made by national brands in my country, are phase to neutral only. Which of them are doing the right thing, that's what I want to know.

And just for you to know, not all APC UPSes come with a site wiring fault LED, mine doesn't and many of their UPSes made for 220-240V countries do not come with this indicator either. But I think this is not the issue since the MOVs will still be connect from neutral to ground.

Thanks once again!

Message was edited by: rau

[voidstar_apc]

The safety aspect of the question seems pretty clear cut: a good ground will not conduct current through you at surge voltages. The cold water pipes have insignificant resistance and are buried in the earth, and therefore should be at ground potential. Potential from the surge would show up on the MOV itself and the ground wire going to the pipe. The thermal fuse in the surge protector prevents it from shorting too long and starting a fire.

You also questioned why APC protects against common mode surge when doing so has the potential to created a ground-neutral bond. It's a very good question, so I had a discussion with John and Mike. John is one of APC's safety compliance engineers, and blows up surge protectors for a living. Here's what he had to say:

John:This is a big issue. There are misconceptions mixed with truths in those articles that the person is referencing. Basically it seems like the person has two main worries. Will shunting surges to ground elevate safety ground and hurt me? Also MOVs fail short and will continue to conduct energy and I wont know about it. I would argue that no protection in the ground mode will make the case of a piece of equipment more dangerous not safer.


Shunting to ground is what you do with surges. It is the safest place to put them. Where does the lightning rod on a house go? If there were no protection in the ground mode a surge of up to 6kV could occur and now you would have 6kV from Line to ground. Problem is even on double insulated equipment there is only 3kV of dielectric isolation to the case. That surge could directly couple to the person touching that case. If you put a surge strip in between the equipment and the outlet. Now the surge will only get to 330V. That will not break down the insulation between the case and the line conductors so the person would not get zapped by touching the case.


What happens to the energy that was shunted to ground? Would it not want to go through the case and the person touching it if we put that voltage on ground? No, it wont. Electricity wants to go back to were it came from. In the case of a potential between Line and Ground, it wants to go back to ground. That ground is in the electric panel where it is bonded to Earth. Electricity always follows the path of least resistance. In this case the resistance from the surge strip down the conductors in the wall is much much less than the resistance from the surge strip to the equipment, through the person, then to ground. None of the surge will go in that direction.


Do MOVs fail over time? Sure they do. But their life is most likely longer than the person would keep the surge strip. Even if it did fail, there are fuses in series with the MOV that will take it out of circuit incase of a failure.

John also explained that although ground and neutral are bonded outside the building, most surges originate from inside from equipment, loads turning off and on, switching supplies, etc.

I also spoke with Mike, who is a power electronics engineer, about the effects of common mode surges on equipment such as power supplies. Like John, he mentioned that common mode surges can exceed the insulation provided by the case and can be hazardous. He also pointed out that the input and output of a power supply are electrically isolated, and so the output has a virtual ground that is at ground potential. That means in the face of a common mode surge, the isolation electronics like the transformer and the optocouplers that bridge the input and output side of the power supply are now facing a fast 6kV potential difference. Furthermore, he explained that during his time at Digital designing power supplies, common mode surges were one of the few things that could glitch the output of the supply.

I hope this answers your questions. It's certainly been educational for me.

[Anonymous user]

Voidstar, I really appreciate the attention you're giving to my concern, but I have a few things I'd like to point out.

On APC UPSes there is no thermal fuse to disconnect an MOV! At least those I have opened their case I couldn't find one. And they have MOVs connecting Neutral to Ground! Surge Protectors made by APC do have thermal fuses, but not the UPSes!

I understand surges "want" to go back to ground and isn't the neutral wire going to that rod on the ground outside your house and the others spread throughout the power grid?? But the neutral wire is not attached to metal parts as the safety ground wire (the green one) is in Power Supplies, computer cases, kitchen appliances, a buch of other electronics and also electric showers. Why is there the need to shunt surges to two wires that are both supposed to be atached to the same rod?

As far as I know, surges can not enter your house via the neutral conductor since surges want to go to ground and follow the easier path! I insist, isn't the neutral wire attached to the rod on the ground outside your house?? I ask again, why is there a need then, to shunt the energy to the safety green wire from inside the house? **Surges generated outside can not come inside your house via the neutral wire** / **Surges generated inside the house is shunted to Neutral, or should be shunted to neutral only, since these are Normal Mode surges and not common mode ones that I once again remember - they do not enter your house because the neutral wire is "grounded" at the entrance.

I understand now that is very unlikely for the energy being carried by the green safety wire to electrocute a person touching exposed metal parts since electricity wants to go to ground and not you that "offers" electricity a much more resistance path to earth, but I still don't accept the fact of using the safety wire for shunting surges. Some may say two paths for diverting surges is better than just one, but in this case these two paths goes to the same place, the same rod outside your house and wouldn't it make much more sense to use a heavier gauge neutral wire then using two "thin" wires - the neutral and the safety green wire? I've read somewhere I can't remember, that when you divert surges to these two wires, there is not a big voltage reduction of the surge that would reach your electronics, they're not divided in half between these two wires, I don't know if this is true.

And to finish with, I go back again to the MOVs attached from neutral to ground that are not protected by thermal fuses. At least the codes in United States don't allow neutral-ground bonds to be created inside a building and there must be a reason for that, one of human safety concerns. So since on many UPSes there is not a device placed to disconnect failling MOV, I think we're still dealing with a safety issue here.

Thanks very much Voidstar, John and Mike!

[voidstar_apc]

I'm not familiar with those UPSes so I would need to ask the designers how they deal with a faulty MOV.

Why not shunt to neutral instead of ground? As John explained, common mode surges cannot be shunted to neutral. Ground is the safest place. Although common mode surges won't enter your house... they get shunted on the poles to ground and neutral is tied to ground... there's still the problem of surges originating inside your house, and unlike your sources, John does not believe they are uncommon. Mike observed tangible evidence of the problem during his time at Digital: power supply failures due to common mode surges. And lastly, the reason for ground-neutral connections not being desirable is because other failures can make that dangerous. To use an example from my first post: a poorly connected ground.

So which do you want? An unprotected device where a common line surge could be hazardous and destructive, or a protected device with a MOV shorted leading to an undesireable ground-neutral connection that shouldn't be hazardous or destructive? In the intro to John's message, he clearly favors protection as the safer alternative.

[Anonymous user · Accepted solution]

You say:

"As John Said, common mode surges canot be shunted to neutral. Ground is the sagest place"

Well, where does the neutral wire goes to?

A: To ground! The safest place! And the neutral wire that goes to your wall outlet is in theory a much better ground since it is also grounded under the pole which the power transformer is attached to and other point along the power grid.

But then you say:

"Although common mode surges won't enter your house... they get shunted on the poles to ground and neutral is tied to ground..."

A: The surges coming from outsideon the neutral wire in common mode is not shunted, they naturally follow the path to earth, there's nothing diverting it, unless it comes on the phase wires. And yes, neutral is, our should be, tied to the ground the goes to you wall outlet via green wire, but even so, common mode surges follow the shortest path to ground and in this case the shortest path is not your wall outlet, but ground! So I can't understand your point here.

Surges generated inside your house, as far as I know, will never be common mode in nature, but normal mode ones, so they're always gonna be shunted to neutral, which is the functional ground wire, not safety ground wire, you can ask Jonh that.

How could common modem surges be created inside your house? Unless a lightning strikes the roof of your house, I can't see a situation where neutral wire inside your house is "naturally" loaded with a surge.

There is also a school of thought that says shunting surges to the safety ground can demage interconnected equipment, especially data communication ports since these devices make a zero volt reference using the safety ground. I don't know how true this is on real world, but there are reports of ports being demaged by surges diverted to the safety ground. In my house there are three interconnected computers and only one is connected to the network protection provided by an APC product, but the other two is not, so I wonder what would happen to the NICs of these computers in case of surges being diverted to the safety ground...

Message was edited by: rau