sec is possible
before the device would trip. Maybe a minor point, but in the past I have
had to caution workers that were using power tools at heights that the
initial shock may cause them to lose balance or drop the tool both which can
have serious consequences.
There are many types of GFCI's from breakers to receptacles to pigtail
plug-in. All sense a current imbalance using a single core CT. So I don't
think the polarity matters, although I've never tired it.
I did run across a problem in the field when a two pole GFCI breaker was
installed on a 240 V circuit. The two pole plug-in breaker has provision
for connecting two hot and a neutral (3 terminals). On the plug-in end is
the two hot clips and a white pigtail. Since there was no neutral the
electricians just taped the pigtail instead of attaching it to the neutral
bar. Everything worked fine except that the test function failed. After
replacing the breaker a couple of times they decided to get a second
opinion. As it turns out the test button routs 5ma of current from the line
side of the CT to the load side neutral (white pigtail) So even if you
don't have a neutral conductor you still need to attach the neutral pigtail
to complete the test circuit.
Thanks for this info.
Can I ask a supplementary along the same lines.
I've heard that a PC should never be used on a non-grounded socket even if
it is protected by a GFCI. Is this true, and if so, why?
A few years ago,my PC was not properly grounded (poor contact of the
receptacle)and touching it I got a shock.
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PC power supplies contain RFI supression components, which as
a side effect leak a small current to ground (too small to trip
a GFCI). The power supply therefore expects that it has a ground
connection through which this current can leak. If it doesn't,
the ungrounded casework will float up to mains voltage (typically
it sits somewhere around half mains voltage, but that depends on
the quality and tollerance of the RFI components).
There are a number of issues with having a piece of metalwork
sitting at such a voltage. If the RFI components are in good
condition, the current available should be less than a 1mA
(at least in the EU -- I don't know what the US regs say). So
risk of shock isn't high, but the RFI components are not designed
failsafe and a shorted capacitor (which could easily be triggered
by a mains voltage spike) could easily leave the case with a low
impedance connection to the live supply.
Connecting peripherals to such a PC has issues. For a Class II
(double insulated) peripheral, it will be carried to mains level
potentials via its data/signal connection to the computer unless
it contains full isolation on the interface. For a Class I
(grounded) peripheral, if it also has no ground connection, then
it will also be leaking a similar current, so the available shock
current available is now doubled. If it is grounded, then the
signal/data connection between the two devices is likely carrying
the leakage current from the PC. Depending on the construction of
the connectors and the electrical protection on the interface,
there are circumstances (particularly live plugging) where this
current and voltage level could damage one or both appliances.
If a ground fault happens in the PC, then the interface will
carry the fault current, in so far as it is capable of doing so
without burning out.
Finally, relying on a GFCI for your protection is not a brilliant
idea. They are themselve prone to non-failsafe failures.
In the UK, we don't allow grounded appliances on non-grounded
outlets at all.
So, if you do this, it will probably work, but there's plenty
of rope available to hang yourself one way or the other.
Primary reason is for safety - if the PC case is made of metal, then
it must be bonded (connected) to safety earth ground to prevent
possible electrocution in case any live loose wire inside the PC
should touch the metal case.
PC power supply is very noisy, as well. It uses a noise filter which
dumps high frequency power supply switching noise into earth ground
(it could be as high as a few milliamps for real noisy power
supplies). If earth ground is not there as a "dumping ground" for
noise, then strange malfunctions (intermittant) might occur.
A laptop IS a PC, which automatically makes the statement
under discussion a myth: "a PC should never be used on a
non-grounded socket even if it is protected by a GFCI."
But read on - I'll address both laptops (with plastic
cases - irrelevant as will be shown) and PC's that
are not laptops, both with and without plastic cases.
My Dell minitower has a plastic case, and a 3-prong plug.
Ground has nothing to do with whether either my ThinkPad
or my Dell work or don't work. The fact that a PC's
electronics work just fine with no ground is proved
millions upon millions of time every day by the work done
The ESR stuff people mention is not well thought out.
Many semiconductor chips are so sensitive that just
handling them can kill them, due to static discharge into
the chip. BUT - seal them inside a conductive UNGROUNDED
material - that's how they are shipped - and they can be
handled with no problem. The static charge is spread via
the conductive material over a wide area such that it won't
harm the chips, even though they are touching the material
and thus very close to your hands and the source of the
static electricity. A PC, in a metal case with NO ground has
a MUCH MUCH greater area for the static charge to spread over,
AND the chips are no where near in contact with the case.
There is no discharge from the case into the chips. The case
simply does not have to be grounded to be effective against
static discharge. If you remove the chips from their
conductive package, or if you remove the case of the computer
to handle the electronics inside, then you need to use
a grounding method to ground yourself before touching the
Think about it. The PC arrives at your home, shipped with
styrofoam packing. If it arrives at the time of year when
there is extremely low humidity, it is obvious that there
is an awful lot of static electricity. The PC cannot be
grounded when it is inside the box, and you need to remove
all the packing to get to the PC. You are handling an
ungrounded PC extensively, more than most people will ever
handle it again, in a high "static environment" - possibly
the highest static environment it will ever be exposed to -
and no static electricity damage results. Ever.
The ground is there for safety. In the event of some internal
failure, it might be possible for the exposed metal parts to
become energized. If that happens and the case is grounded, you
won't get hurt. That is the purpose for the ground - safety.
If the PC is plugged into an ungrounded GFCI receptacle, and
the same failure occurs, the GFCI will trip, preventing you from
Someone pointed out that a GFCI could fail. That's certainly
possible, and so one can say, based on that, that it is
*better* to use your PC on a grounded receptacle versus
and ungrounded GFCI receptacle. But that possibility does
not support the myth that one should *never* use a PC
on an ungrounded, GFCI protected circuit.
in article gqKKb.23656$R email@example.com, Den Murray at
firstname.lastname@example.org wrote on 1/6/04 6:46 PM:
This is true. The reason is to protect the silicon devices from ESD,
electrostatic discharge. If a sufficiently large potential difference is
generated by friction or charge transfer, little energy is needed to get a
permanent breakdown failure in silicon devices, particularly MOSFETS with no
built in charge leakage. GFIC is to protect people, not mineral devices.
If the equipment has a ground connection, many circuits purposely provide
leakage paths to ground to prevent the buildup. Without a ground, that does
You are confusing polarity with line and load.
It really doesn't care about polarity although the code does.
The thing you do have to do is be sure the line side is connected to the feed
and the down stream load is on the load terminals.
The proposed U/L spec is that they punish you ( Won't reset?) if they are
swapped. I imagine they could make them work if they were wired either way but
I bet they won't be $10 anymore.
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