Any ideas about why bathroom exhaust fan in shower trips GFCI outlet

Status
Not open for further replies.
iwire said:
Explain 'inductive kick back'.
Click to expand...
Simple explanation is the energy surge sent back through the lines when the magnetic field in an inductive component of the circuit (the motor) collapses - usually when the current is interrupted somehow.

It is the basis of how they create a spark in the ignition systems of older internal combustion engines that used "breaker points" to trigger the spark.

Your well known brands of GFCI's have their circuitry designed to withstand a certain amount of kickback without tripping, but many cheap ones have more nuisance tripping issues and part of it is because this inductive kickback is not filtered, or not filtered well enough.
 
140501-0903 EDT

iwire:

Energy is stored in the magnetic field of an inductor when current is flowing thru the inductor. The current thru an inductor can not be instantaneously changed.

Consider a series circuit of a battery, switch, and an inductor. An automotive ignition system is such an example.

Close the switch and the current gradually builds to a steady state value defined by the source voltage and the total series resistance.

Now open the switch and instantaneously after opening the switch the current remains the same, but the series resistance jumps to near infinity. Thus, the voltage across the switch jumps toward infinity, but the air gap between the switch contacts breaks down limiting the peak voltage. In typical situations this voltage can range in the thousands of volts. That is your inductive kick.

I use a magnetic ballast 8' Slimline as a transient voltage generator. Rapidly wiggling the plug in a socket produces several thousand volt transients.

In an automotive ignition system a capacitor is placed across the breaker points. This creates a resonant circuit that limits the rate of rise of voltage across the switch and limits the peak primary voltage on the ignition coil to around 1000 V. This 1000 V was obtained from the energy from the 6 or 12 V battery that was put into the inductor during the charge phase (switch closed). The 1000 V on the primary is stepped up to about 30,000 on the ignition coil secondary. But this voltage is never reached because the spark plug breaks down at around 10,000 V at full compression.

.
 
140501-0943 EDT

iwire:

On opening the path of current flow to an inductor, by virtue of the fact that the instantaneous current thru an inductor cannot change, the inductor becomes a voltage generator as the magnetic field tries to collapse and produces a voltage high enough to cause an arc at the switch contacts and then current flows. The same magnitude of current as before the switch was opened. This all occurs in an instant of time.

In the real world capacitance exists in any circuit and changes are not as quick as an instant.

How can current flow in an open circuit? Your question. Current is a measure of the rate of flow of electrical charge (electrons, protons, holes, or whatever). A capacitor is basically an open circuit on a steady state basis. However, it is possible for charge to flow to one plate and away from the other plate, and not pass thru the dielectric in the capacitor. In the real world this only lasts for a period of time defined by source voltage, series resistance, and breakdown voltage of the capacitor. In an AC circuit charge can flow back and forth continuously.

.
 
With an AC circuit, doesn't the charge in the inductor become the polarity of whatever point in the AC cycle was when the switch was opened, and therefore the discharge of stored energy in the inductor will be DC with polarity dependent on when the switch was opened. Then the discharging will occur either by jumping gaps if voltage is high enough to do so, or through capacitive coupling with the circuit and it's surroundings.

ETA: the bulk of the discharge likely being through arcing at the contact that is opened, the remaining charge is dissipated via capacitive coupling.

Just my thoughts on what likely happens, go ahead and correct me if that is wrong.
 
Last edited:
140501-1008 EDT

iwire:

If I have an electric field and place an electron in that field, then that electron will move and that is an electric current. I can have an electric current flow in a vacuum (no conductor).

This can be illustrated by an electron beam emitted by a cathode in a CRT and flowing thru the vacuum to the faceplate of the CRT. In this case the electrons are charged particles in free space shaped into a concentrated beam by electric and magnetic fields, accelerated to the face plate by an electric field, and deflected by magnetic and electric fields.

.
 
gar said:
140501-0943 EDT

iwire:

On opening the path of current flow to an inductor, by virtue of the fact that the instantaneous current thru an inductor cannot change, the inductor becomes a voltage generator as the magnetic field tries to collapse and produces a voltage high enough to cause an arc at the switch contacts and then current flows. The same magnitude of current as before the switch was opened. This all occurs in an instant of time.

In the real world capacitance exists in any circuit and changes are not as quick as an instant.

How can current flow in an open circuit? Your question. Current is a measure of the rate of flow of electrical charge (electrons, protons, holes, or whatever). A capacitor is basically an open circuit on a steady state basis. However, it is possible for charge to flow to one plate and away from the other plate, and not pass thru the dielectric in the capacitor. In the real world this only lasts for a period of time defined by source voltage, series resistance, and breakdown voltage of the capacitor. In an AC circuit charge can flow back and forth continuously.

.
Click to expand...

So are you are suggesting that the very small motor in a bathroom exhaust fan is generating enough power after the switch is open to both jump open contacts and create more than 6mA of current flow causing the GFCI to trip?



I find that highly unlikely, I see much larger motors supplied from GFCIs that are cycled many times per hour with no GFCI trips.

I find this inductive kick reason for GFCI tripping on par with 'dirty power' and addtional ground rods needed as reasons for corrupt data transmission.
 
That would also make sense why contacts are rated differently for inductive loads then the same contact is rated for a resistive load, which also gets us to why switches need to be rated in horsepower and not just amps when used for motors.
 
iwire said:
So are you are suggesting that the very small motor in a bathroom exhaust fan is generating enough power after the switch is open to both jump open contacts and create more than 6mA of current flow causing the GFCI to trip?



I find that highly unlikely, I see much larger motors supplied from GFCIs that are cycled many times per hour with no GFCI trips.

I find this inductive kick reason for GFCI tripping on par with 'dirty power' and addtional ground rods needed as reasons for corrupt data transmission.
Click to expand...
I can not tell you exactly what happens here to trip the GFCI's, hopefully maybe gar can clue us in, but I do know that one of the issues with early GFCI's was inductive kickback, they have solved that issue with reputable brands, a severe enough case of kickback may still cause some trips, but probably not happening with a small motor like we have on a typical bathroom exhaust fan.

Some suggestion as to what may be happening - if the load is switched with a single pole switch - some of the kickback may be unbalancing the GFCI as it is dissipating any remaining energy after the arc is extinguished through the unswitched neutral conductor, after all it is a longer conductor when the switch is open so it will have more capacitive ability, use of a double pole switch could possibly reduce "nuisance tripping" in such a case.
 
And again I point out that many motors operate on GFCIs every day.

I would be looking for a more direct cause of this GFCI issue, like failing insulation, neutral to grounding contact etc. For the price of a fan motor I might just swap it out.
 
iwire said:
And again I point out that many motors operate on GFCIs every day.

I would be looking for a more direct cause of this GFCI issue, like failing insulation, neutral to grounding contact etc. For the price of a fan motor I might just swap it out.
Click to expand...
Understood, but if the tripping is happening only when the load is switched off it is very likely inductive kickback is causing the tripping, but still could be that this kickback is finding a discharge path via a failing motor winding or other weak insulation point. But kickback on a bigger motor may not be anything wrong with the motor just that the GFCI is not designed to accept that level of kickback.
 
140501-1333 EDT

iwire:

In response to your post numbered #27.

Inductive kick is not the cause of the problem in this thread. It was already established that the GFCI trips after turn on and before turn off of the circuit.

Inductive kick and transient voltage tripping of GFCIs was a side subject that popped up.

The inductive kick cause of GFCI tripping is likely caused by defective design of a GFCI or an extremely severe transient voltage. Probably cause is not an unbalanced current.

In a post quite a long time ago I described tests on a Leviton GFCI where I could produce transient voltage tripping of the device by rapid switching of my test 8' Slimline fixture on the output side of the GFCI, but not on the input side. I can see an obvious layout problem on the circuit board that could provide capacitive coupling to the gate circuit of the SCR that operates the trip coil.

Whether the small fan motor described here can produce switching transients to cause some GFCI to trip I do not know.

.
 
Inductive kick as a cause for GFCI tripping ( as Gar stated not necessarily in this posters case)
and many other electronic failures is well known by electrical engineers with EMC experience and GFCI manufacturers also.

Post #27 :happysad:

Google Electric Fast Transient
 
I truly respect the engineers here but at the same time I try to keep things simple. It seems some try to find a complicated reason for a basic problem, personally I find that non-productive.

I am going to remain unconvinced that 'inductive kick' has anything to do with the OPs issue.
 
If indeed the fan is the culprit then change the fan. I would not spend much time on it but make sure the fan is the issue.
 
iwire said:
I truly respect the engineers here but at the same time I try to keep things simple. It seems some try to find a complicated reason for a basic problem, personally I find that non-productive.

I am going to remain unconvinced that 'inductive kick' has anything to do with the OPs issue.
Click to expand...
Call me a bad boy if you want.

Back in post 14 it was mentioned that someone once had a similar fan that did trip a GFCI when it was switched off. I replied and said in that instance that trip was most likely due to inductive kickback, or at least that is what I was trying to say. I did not mean it to be interpreted as the cause in the OP's case. But we then got kind of off the original topic after that and into the inductive kickback issue.
 
140502-1731 EDT

An experiment. DC power source, a small transformer primary for an iron cored inductor, a 0.01 ufd capacitor in parallel with the inductor, and two wires for a switch. Charging current to inductor 200 mA, and about 10 V drop across inductor.

Peak voltage across the inductor upon opening circuit was -1200 V and peaked at about 150 microseconds. Note that this is a reversal of the polarity of the charging voltage as is expected. The peak voltage was 120 times the steady state voltage across the inductor. However, charging current is the more important factor in the output voltage because the current determines the energy stored in the inductor.

.

.
 
Status
Not open for further replies.
Top