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In my science classroom all outlets on the lab benches are gfci protected--that's great. I plug in multiple computers into some of these outlets and use power strips to do this, but it seems there might be a problem occasionally with this.

It seems that there might be a negative interaction between the surge-protected power strips and the gfci e.g. excessive tripping of gfci or even wearing out of the gfci. The electrician told our head custodian that it wasn't good to use surge-protected strips with gfci and I'm wondering if this is the problem and if so is there some way to mitigate it. But, maybe it's just a bad strip, maybe these old computers' power supply?

Edit: Before posting this question I spent ~30 minutes searching E-SE site for any information on possible interaction between these two components, to no avail. These 2 components are commonly used in residential and commercial electrical power distribution systems, yes? Ideally, the answer will provide specific technical knowledge on the design/functioning of these 2 components and explanatory/theoretical knowledge explaining why they might interact negatively.

HOWEVER, my question assumed 'generic' designs of surge protection circuits in inexpensive power strips and also generic designs of gfci. Perhaps this is a bad assumption?

mmathis
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David G
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7 Answers7

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Actually residential surge protectors have 2 metal oxide varistors, 1 from hot to ground and 1 from neutral to ground. MOV's work by dumping excess voltage to ground. After taking several hits some MOV's start conducting to ground at lower voltages until they fail. MOV's in power strips are after the ground fault protective devices (GFCI's) so if they do there job during a spike or after they are damaged from doing their job they can trip a GFCI. The way to eliminate false trips and still protect sensitive equipment is to install a "whole house" or main panel surge suppressor in the main with a dedicated standard inverse time double pole circuit breaker as required by the manufacturer. This will dump spikes on the line and from other branch circuits that may generate spikes but will not protect fully if a motor load is on the same branch circuit. If motor loads are on the same circuit a plug in or power strip surge suppressor may be the only way to protect the equipment but once they start tripping the GFCI protective device they will need to be replaced.

Ed Beal
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You all gave good answers. A GFI will trip with as little as 10 milliamps to ground. Since an mov can conduct 5000 amps to ground during a spike it can easily trip the GFI. all it takes is a small power Spike from something turning on or turning off like the AC unit. Gfi's are not compatible with surge protectors is the best way to sum it up. Now you could plug a portable GFI into the surge protector and it would work just fine. The surge protector has to be before the GFI. Gfi's protect humans from electrocution by removing the hot if even a small amount of current goes to ground.

Reuben B Smith
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If either the power strips or connected equipment have capacitors between the power conductors and ground, there would be some current to ground through the capacitors. If there are MOVs between power and ground, there could also be some leakage through those. Deterioration of insulation can also result in small intermittent leakage currents to ground before the insulation actually fails and trips the GFI or breaker every time the faulty device is plugged in.

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"A" (Amps) is amount of current and "W" (Watts) is a calculated value of V (Volts) * A. Lets calculate a typical low powered desktop PC usage at Max values. Desktop Processor (65W), HDD (9W), mouse/keyboard (1W), 22" LCD monitor (37W) = 112 Watts. Now lets break that down into amps. V*A=W or W/V=A since I live in the US where we use 110V circuits, 112/110=1.0. So with one "typical computer" being no more than 2 Amps (to be conservative) and most circuits here in the US are rated for 15A it would be safe to have at most 7 full computers on one circuit and not worry about tripping it even if you didn't know what it was rated for. Further more, a power supply will not draw more than what the devices that it supplies power to are asking for (using a 1200W power supply on this typical desktop PC will not draw more than 80W). The "1200W" is just noting what the maximum power of the culmination of all downstream component can draw. also, a phone charger may say that it is supplying 3A but you are missing the 5V marking making that charger only safely being able to supply 15W. divide that into amps on a 110V circuit and you will see that you could safely plug in hundreds of those into one outlet and be completely safe of not tripping the 15A breaker.

Richard
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Surge protectors typically have three MOVs -- from hot to safety ground, from neutral to ground, and between hot and neutral. The connections to safety ground, if they leak any current, would indeed be the kind of unbalanced flow that the GFCI is designed to detect and cut off.

A properly designed surge suppressor shouldn't leak enough current to be a problem, under normal circumstances. However:

1) Any surge being shunted aside might cause the GFCI to pop. That's a case of everything working as designed, even if leaving the equipment unprotected by a surge suppressor would not blow the GFCI (but might blow the equipment).

2) MOVs do eventually take enough surges to start to fail. I'm not sure what the failure modes are, but if one of them starts leaking more than its spec calls for that too could trigger the GFCI. Again, though, that's the GFCI working as designed and telling you that the surge suppressor is no longer trustworthy.

So I don't see any reason not to try this combination. If it works, it will continue working modulo surges.

keshlam
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It can be an accumulation of multiple devices.

Not enough from a single device but given you are using power boards, it is the sum of many small leakage currents on one circuit.

Should be possible to fault find, try substituting out the power boards for new or non surge protected model.

Also try using less computers and devices on one circuit. It may be an accumulation from multiple devices or one in particular may be causing issues.

Narrow your problem down.

D-on
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Most folks here are ignoring the obvious and over thinking this,its simple math.There are also not enough details for a clear conclusion.

You should not be plugging more than 1 PC into any outlet, ESPECIALLY on a power strip or GFI.

Most GFI outlets are rate at 20 amps? Even a low end PC at Idle will pull at least 10, and x2 PCs under heavy load up to 25 amp peaks w/ x2 monitors.

David Rebar
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