3

As I went looking for a replacement for a GFI wall outlet, I noticed that the advertised features of one outlet included "low voltage trip" – that is, the GFI will trip not only when a ground fault is detected but also when voltage drops below a certain threshold.

As a result, the GFI will trip and need to be reset whenever the circuit breaker trips, when there is a blackout or if you've wired your outlet in series with a switch (as for a power outlet outside the house which you want to be able to switch on/off from inside).

Is there an actual safety gain from designing a GFI outlet that way, or is that just an undesirable characteristic of a particular GFI design?

  • Perhaps to add protection against brown-outs? Such a device could be employed to protect sensitive electronics that may be plugged into that outlet. Are you sure that turning off the circuit breaker would lead to this GFCI device requiring a reset? Perhaps you could state the exact model of the device so that others could locate datasheets and better answer your question. – Joshua Berry Apr 7 '16 at 15:09
  • What's the make and model number of the device? Can you provide a link to the product? – Tester101 Apr 7 '16 at 15:43
  • This is the product (unfortunately I was unable to find an English description of it): produktinfo.conrad.com/datenblaetter/600000-624999/… – user149408 Apr 8 '16 at 7:37
  • Hmm my German is as good as my Russian so I can't read that. However, GFCI's here in the west can't be tripped without being powered. So if the breaker trips the outlet can't trip. I would suspect a trip on low voltage is to protect equipment from being run on low voltage. This can damage some epuipment especially motors that will overheat when low voltaged. – ArchonOSX Apr 14 '16 at 11:34
2

I've come across a scenario in which this feature might actually improve safety:

Some older European buildings still have TN-C wiring in place. Current wiring codes no longer allow this due to safety concerns (which I'll get to in a minute), but there is no requirement to upgrade as long as no modifications are made (simple replacements, such as installing a new switch in place of a faulty one, do not count as modifications).

With TN-C there are two wires from the main panel to the wall outlets: a live wire and a combined ground-neutral (PEN) wire. At each outlet, there is a short length of wire bridging the ground and neutral terminals, and the PEN wire is attached to one of these two terminals.

However, an interruption in the PEN wire presents a safety hazard: devices are still powered via the live wire but the circuit is interrupted due to the faulty PEN wire. Touching the metal chassis of any device on that circuit will have the same effect as touching the neutral wire: doing so while grounded will turn you into the missing link in the PEN wire, which can be deadly.

This was the reason for phasing out this practice in favor of TN-C-S, which splits up the PEN into two wires at the distribution panel. With TN-C-S, interruption of a single wire affects either the ground wire or only the neutral wire, neither of which present an immediate risk of electric shock in an otherwise compliant setup.

Enter the GFI outlet: if used to replace a regular outlet in a TN-C setup, the PEN gets interrupted and someone were to touch the metal chassis of a device plugged into the GFI outlet, the GFI would trip. At the most, you'd feel a slight, short tingle as you touch the device.

Since a faulty PEN is likely to cause a brownout or blackout, having the GFI trip on low voltage would likely cause it to trip on a PEN interruption. Devices plugged into the outlet would not be hazardous to touch (and not even give you a tingle as you did), although devices not behind a GFI would obviously not benefit from this and still present a hazard. Additionally, a tripping GFI may alert people that something may be amiss – another example why a circuit breaker or GFI tripping should not just be considered a nuisance to be fixed by switching the power back on, but as an indication of a potential hazard that needs to be looked into.

  • It sounds like they institutionalized what we call "bootlegging ground". – Harper - Reinstate Monica Mar 3 '17 at 17:59
  • Pretty much, yes. Germany eventually banned this practice in 1973, but in older buildings (unless they’ve been rewired), it’s practically standard. At that time, Germany didn’t mandate GFIs in home wirings at all, and when they were made mandatory, initially that was just for humid areas (mostly bathrooms)—there was no requirement for a GFI on every circuit until around 2008. – user149408 Mar 4 '17 at 23:59
1

Today I stumbled across another related topic on German Wikipedia:

There is another type of device called SPE-PRCD (Switched Protective Earth-Portable Residual Current Device), which is essentially a GFI built into a device (or the power cord of a device) which can be plugged into a conventional power outlet. In addition to tripping on a ground fault, these devices perform various tests to ensure the outlet is wired and grounded correctly.

These devices also trip when voltage is low, requiring them to switched back on manually after a power outage. This is explained as a security feature: When using a power tool (electric saw, power drill or other) and there is a power outage, users may forget to switch off the tool. If power is then restored, the tool will spin up unexpectedly, potentially causing injury. Forcing users to manually switch the device back on after an outage prevents injuries resulting from power tools spinning up unexpectedly when power is restored.

Not sure if that was a consideration for that particular GFI outlet, but it is likely that the engineers thought it would be a useful feature for an outdoor power outlet which might be used for lawn mowers and the like.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.