If you live in North America, you’re probably familiar with electrical outlets that have
a test and reset button.
Older ones would have red and black buttons, but newer ones are usually matched to the
color of the receptacle.
If you’ve ever messed about with one of these and pressed the TEST button, you’ll
have noticed it sort of POP with the RESET button sticking out a bit,
and now the outlet is dead.
To be fair, it told you to test it.
But now you have to exert quite of bit of force to shove that reset button back in place
before the outlet will work again.
What is this for?
Why are they usually only found in kitchens and bathrooms?
Will I ask a fourth question?
And how are they, as the title suggests, life-saving?
Well, this is called (in Americaspeak) a ground-fault circuit interrupter, or GFCI.
Sometimes they’re just called Ground-Fault Interrupters, or GFIs.
The rest of the world calls them Residual Current Devices, or RCDs, and usually they
aren’t found in the bathroom but in the service panel protecting the entire circuit
(and sometimes the entire house).
These simple devices use a fundamental principle of electricity to detect when an electric shock
might be in progress, and can nearly instantly cut power to the circuit to stop
said electric shock.
The US electric code requires these outlets to be fitted when they are within a certain
distance of a water source.
That’s why they’re usually found in the kitchen and bathrooms, though electrical outlets
found in other potentially wet locations, such as exterior outlets or those in a laundry
room or garage, will usually require protection as well.
The theory is that you’re much more likely to experience an electric shock near water,
‘cause water tends to conduct electricity pretty well and thus if your hands are wet
or a power cord is wet, you’re at a significantly higher risk of electric shock when touching
anything remotely electrical.
Anyway, how do these devices determine if a shock might be happening, and thus how do
they know they need to break the circuit?
Well, part of the answer is in the name.
The Americaspeak version, ground-fault circuit interrupter, suggests it can detect some problem
related to the ground.
The most-other-places name, Residual Current Device, suggests current is going somewhere
it shouldn’t.
I’ve always felt that both of these names compliment the other and make the issue easier
to understand, but on their own they’re somewhat inadequate.
Residual current is kinda the result of a ground fault, but what does that even mean?
Well, in any ordinary circumstance, the current flowing out of one side of the outlet will
exactly match the current flowing back into the other.
There should always be a balance in an electric circuit
between the hot supply and the neutral return.
If you plug in a toaster, then for every unit of current flowing towards the toaster in
this wire, there is an equal unit of current flowing away from it in the other.
The same holds true for the reverse polarity of the A/C cycle.
But if I were to get an electric shock from the toaster, perhaps by being a complete fool
and sticking a knife down there like you should never ever do, kids, then some of the current
coming from the outlet gets diverted through my body.
Now, the current leaving the outlet is greater than the current returning,
because some of it doesn’t actually return.
There is now an imbalance between the current flowing out of the hot wire and back through
the neutral wire.
This fault condition is, from the outlet’s perspective, a ground fault.
Some of the current is not returning to ground, or the neutral side.
Somewhere outside the circuit, there is residual current.
See, both names work, but they describe the problem differently.
With a ground-fault detected,
With residual current detected,
the device needs to
interrupt the circuit.
So it’s a ground fault circuit interrupter.
So it’s a residual current device.
As a side note, I like the term ground-fault circuit interrupter better because it describes
both what it detects and what it does.
Even the 25% discount term, Ground Fault Interrupter, describes both the problem and the action.
Residual current device is a little incomplete in describing its mission, but I will grant
that residual current seems like a less-technical description than ground-fault.
But whatever.
Now, I’ve always found the best way to show how devices do what they do is to tear one apart.
Off to the hardware store!
I’m back!
So let’s take a look at this thing.
Like any electrical outlet, is has terminals for incoming hot and neutral, as well as a
separate ground.
But these ones here are a little interesting.
See, most standard outlets have two pairs of terminals as well, but they’re connected
by this little tab.
This electrically joins the two halves together, so connecting just one pair of wires powers
both sides of the outlet, and you easily can daisy-chain outlets together within a circuit.
However, if you break the tabs off, now each outlet is wired separately.
This is often done so that one pair of plugs can have two functions, with one side on a
light switch for a lamp, and the other on all the time.
But the second set of terminals on a GFCI is protected by its internal circuitry.
That’s why they are labeled LINE and LOAD.
Incoming power goes into the LINE terminals, and any outlets farther down the circuit that
are attached to the LOAD terminals will also become ground-fault protected.
The practical upshot of this is that in a chain of outlets on one circuit, only the
first needs to be a GFCI receptacle.
The rest downstream will all get protection, though there is a limit to how many you can
string along depending on national and local electric codes.
One little curiosity is that most outlets of this type can interrupt 20 amps, so although
this is only a 15 amp receptacle, it can be placed in a 20 amp circuit and provide protection
for other 20 amp receptacles.
So, let’s open it up.
With everything removed we find the four screw terminals mounted to a circuit board.
These braided copper wires are carrying current from the line side through to the load side,
and the top pair of switch contacts would normally energize the pins of the actual receptacle,
which when assembled lie far above the circuit board.
This nylon bracket can move back and forth, and it forms the actual switch that will break
the circuit in a fault condition.
It rests in the closed condition with the help of a latch, and a spring down below will
keep it in the open position once enough force is exerted on it to overcome the latch.
Now, this black cylinder piece is a tightly wound coil of wire called a solenoid, and
when current is passed through it it creates a magnetic field which will force an iron
plunger out of it in this direction.
This plunger isn’t visible but it is what breaks the circuit.
When the electronics detect a ground fault, they divert power into the solenoid which
will push the plunger forward and thus kill the power.
But how does it detect current leakage?
Well, look closely at the path the electricity takes from the line connections through to
the switch contacts.
It goes via these busbars through a round doo-dad, and if we move this varistor out
of the way we can see that inside is a coil of wire.
This is the sense coil, and if you look on the bottom you find that this is what is being
monitored.
You can see that IC1 has its pins connected to the output of the coil, with some support
components peppered in.
And now we go back to school for a moment.
You were likely taught that when current passes through a wire, it generates a magnetic field.
Likewise, when a magnetic field encounters a wire, it induces a current in the wire.
Basic stuff, but this is exactly the principle that makes the GFCI work.
See, in a normal condition, whatever unit of current is going up through this side is
also going down through that side.
The current going to the toaster as before, goes up this side, and the current coming
back from it goes down that side.
Of course that’s constantly switching back and forth due to the fact that we’re dealing
with A/C electricity, but they are always opposite directions.
Both bus bars generate a pretty sizeable magnetic field around them depending of course on the
load, but because they are going in opposite directions the fields cancel each other out.
That means that normally, no current is actually induced in the sense coil.
Even though there are two magnetic fields being generated, they are of equal amount
and opposite polarity, so the net result is zero.
But if there’s any imbalance at all between the current going up one side and down the
other, now the magnetic fields are no longer in equal opposition and they don’t entirely
cancel out.
A tiny imbalance generates enough current in the sense coil for the electronics to detect,
and as soon as they do so the solenoid fires and disconnects the circuit.
Most devices like this are designed to break the circuit in 30 milliseconds or less, and
in the US they are designed to trip with only 5 milliamps of leakage current.
So what’s the real-world use of this?
Let me show you.
A word of caution for the following demonstrations.
What I’m doing is pretty dangerous.
Energizing exposed terminals at line voltage is not something you should casually do.
Let me do the dangerous stuff, and please don’t try this at home.
I’ve wired up this naked GFCI to a plug and I’ve put a few of things on its output.
First, a standard light socket with a standard bulb.
Second, the same light socket but with an adapter for an itty bitty bulb, and this one’s
wired correctly.
And third, the same light socket and adapter, but this time it’s wired incorrectly.
So right now, everything looks good.
The current exiting the plug always matches the current returning, so the electronics
don’t intervene and the light stays lit.
Now I’m going to screw this little 5 watt bulb into the top light socket.
Nothing happens, it just comes on.
But now I’ll tighten the light on the bottom.
As soon as it makes contact, the electronics in the GFCI intervene, firing the solenoid,
and breaking the circuit.
But why?
Well, the second light socket was wired with a deliberate ground fault.
I attached its hot wire to the monitored output of the GFCI just like the the first one, but
its neutral wire was hooked into the supply neutral of the outlet, therefore bypassing
the sense coil.
This meant that the current that flowed out through this wire and into the bulb didn’t
take the same path back to the outlet.
It leaked out somewhere (in this case just to here), and the outlet could detect the
resulting current imbalance through the sense coil.
Even though this lamp is really small, passing only 41 milliamps when it’s lit, the GFCI
could immediately detect the fault and broke the circuit.
The second lamp is analogous to someone getting an electric shock.
Current flowed out of the outlet, but it didn’t make its way back in.
If this were a human body rather than a light bulb, said human could be in for a
shocking experience.
But thanks to the GFCI, the fault condition was immediately detected and the current flow
was stopped.
Let me show you how fast this happens.
I’ve disconnected the return wire so I can just push it against the contacts.
If I go to this contact nothing happens because the current is taking the correct path to ground.
The current returning from the bulb goes through the sense coil.
But if I just barely brush against the incoming neutral connection, causing the return current
to flow outside the sense coil, it detects the imbalance imperceptibly quickly.
And that’s why these are life savers.
Imagine you’ve plugged your hair dryer into the outlet in your bathroom, and the cord
is frayed.
You might have never noticed it, but if you touched that wire with a wet hand you’d
be in for a nasty shock.
But if plugged into a ground fault interrupter, almost immediately the current flow would
be stopped and your life may very well have been saved.
And that’s why most modern devices that are going to be used in the bathroom,
like a hair dryer,
are required to have a GFCI built into their power plugs here in the US.
There are plenty of older homes without GFCI equipped receptacles,
and for these your-hands-will-definitely-be-wet
scenarios, it’s better safe than sorry.
And now, some other things!
First, in the US, these generally are NOT circuit breakers.
OK, yes they are, but I mean they don’t protect against short circuits
or excessive current.
They are not a replacement for a traditional circuit breaker but are instead a supplement
to them.
They do not duplicate the overcurrent protection of your standard circuit breaker or fuse.
Many GFCI receptacles here in the US have an LED to indicate...
something.
The state that is being indicated is entirely nonstandard.
Many, such as these, have a light indicating that it’s working.
Presumably that light would go out if the protection has failed.
But I’ve also seen plugs where the light is normally out, but comes on when the outlet
has tripped!
And these ones in my kitchen are normally green and are off when tripped, but when you
reset them, they briefly illuminate red.
So probably, they would light up red if the protection circuit had failed.
Which does happen.
That’s why they are all labeled “TEST MONTHLY”.
And the neat thing about the test is that this actually creates a ground fault!
You might have noticed this resistor apparently randomly sticking up from the circuit board.
This resistor creates a path to the incoming neutral, and pressing the TEST button shunts
this resistor to the monitored hot.
So by pressing the TEST button, you are for real testing its ability to detect a ground
fault because you actually are creating a ground-fault internally.
Even better, the resistor is sized to roughly match the minimum leakage it’s designed
to detect.
So definitely test these periodically, especially since leaving them in the non-tripped position
for a couple of dozen years might make them mechanically seized up and prevent them from
doing their job should the need arise.
You might be wondering why we in the US put these in outlet boxes when others put them
in service panels.
Well there’s pros and cons to each method.
Doing it in the US fashion makes it obvious if any installation is up to code, as a lack
of GFCI outlets in a bathroom or kitchen means an obvious fail.
It also probably encourages testing if the device is easy to access rather than being
part of a circuit breaker panel.
However, there is a benefit to having this protection in all areas of the home.
Sure, an electric shock is more likely in wet places, but it’s not like no one has
ever received a shock in their bedroom or whatever.
Plus, in many countries, residual current devices are combined with circuit breakers,
forming one device called an RCBO, for Residual Current circuit-Breaker with Overcurrent protection.
But putting the protection in the service panel makes troubleshooting a whole lot harder.
If something malfunctions and causes a ground fault anywhere in the circuit, you might be
spending a long time determining what device is actually causing the fault.
Putting the protection at the outlet makes it rather obvious.
One thing that I discovered when tearing this apart is that the internal contacts are able
to accomodate a NEMA 5-20 plug.
Normally US devices that require 20 amps will have this plug where one pin is sideways,
thus preventing you from plugging it into a 15 amp circuit.
The fact that this device has internal pins capable of accepting this plug, plus the fact
that it can break 20 amps as most GFCI outlets can, means the only thing preventing this
receptacle from actually being a 20 amp receptacle is the shape of the holes on the plastic faceplate.
Which means that, in the case of this particular model anyway, they charge you $3 more for
the same product with a slightly different piece of plastic on the front.
Yay.
And finally, though these are super helpful at reducing the chance of injury or death
due to an electric shock, they shouldn’t be seen as an excuse to be reckless around
electricity.
They are a very effective safety net, but why risk falling in the first place?
That said, if you’re a tinkerer who likes to work on electronics, installing one of
these in your workshop might be a very good investment.
At the very least, it might spare you the pain of a zap.
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