Insufficient lightning protection for electronic
A friend owns sensitive high quality Hi-Fi equipment. He wanted to
protect it against lightning strike surge peaks. I advised him towards
my favorite local electronics store to buy a lightning protection
device. A few time later his home was struck by lightning and one of
his apparatus got damaged. Probably without the protection system
things would have been way worse. Nevertheless I was quite annoyed the
protection had not been perfect. So I started studying the problem. My
friend's protection device could not be opened so I focused on the
build-in lightning protection of most computer devices. Every computer
I opened contains the same protection components: two blue 470 V
varistors. One between neutral and ground and one between phase and
The role of such a varistor is to become a shortcut should the tension
between its two wires exceed 470 Volts. So the tension between the
phase wire and the ground wire never can exceed +470 V or -470 V. As
well as between the neutral wire and the ground wire. Any tension on
the neutral or phase wire that would be lower than -470 V or higher
than +470 V, whatever the source of that tension pike, whether it be a
lightning strike or a condensator discharge, will be drained to the
ground by the varistors. The current they can let through can be very
high provided it has a short duration. The maximum tension present
between 240 V AC normal mains wires is 339 V. So a 470 V varistor
protection means the tension is not allowed to go 40% higher than the
maximum normal value.
At first glance the protection seems perfect. Yet is is not. Because
what matters for an electronic device power source is the voltage
between the neutral and the phase. If the tension difference between
neutral and ground will be held to a maximum of 470 V and the tension
difference between ground and phase is held to a maximum of 470 V too,
this means the tension difference between neutral and phase can rise up
to 2 x 470 = 940 V.
For example suppose at a given moment the phase tension rises to its
normal maximum of +310 V. A lightning strikes and would tend to make
the neutral wire go down to a tension of -10,000 V. The varistor
between neutral and ground trades in and cuts the neutral wire tension
back to a mere -470 V. This makes the tension between the phase wire
and the neutral wire be 310 - -470 = 780 V.
That is 150% higher than the maximum normal value! A though device will
withstand such a tension pike. Yet a not a sensitive or fragile device.
Something needs to be done to force the tension between phase and
neutral not to exceed say 470 V. The solution is simply to solder a
third 470 V varistor between the neutral and phase wires:
Such a theory has to be tried out. I didn't want to wait for a
lightning strike so I used an old VGA computer display that makes my
computer crash. Sometimes, when I switch that old display on, the ATX
power source of my computer goes into protection and suddenly halts the
computer. This is due to tension peaks on the mains produced by the old
VGA monitor being switched on. The two varistors my computer power
source contains are not sufficient to protect the power source.
Actually they do protect but not enough to prevent the power sources
chooses to switch off. So I put a varistor between the neutral and
phase wires of my computer and tried to switch the old VGA display on
and off many times. The computer power source never switched off. So
clearly a third varistor is a good thing.
Maybe there are technical reasons not to do so. But I did not think or
hear of any serious one till now. A kind reader informs me a reason can
be in some countries the neutral is wired to the ground, so the third
varistor would be useless.
My computer system is now well protected against lightning strikes.
Both the monitor, printer and computer are protected. But when I hear
thunder I tend to unplug the modem. Because the path from the mains
towards the phone line can be a way for a high current lightning
strike. I'm not sure about this but I prefer not to take any risk. So I
unplug the modem. Following the same reasoning I unplug my computer
from the local network. Because there could be a lightning path between
the networked computers. Good network and modem devices contain
protections against lightning. But who knows. I could not check them to
be 100% effective and I'm not ready to risk my computer to test this
out. Finally, I make sure the computer, monitor and printer are on the
same mains wall plug. That way there is no lightning path between two
different wall plugs (for example should the computer be on one wall
plug and the monitor on another wall plug situated another side of the
- Electric wires colors and names displayed in this page may not
apply to your country.
- Do not solder or try out anything on mains wire if you are not
fully qualified. This could result in damage to your equipment, house
fire, injury or death.
- Many components look like a varistor but are not a varistor. And
varistors are made for a wide range of tensions. So unless you really
know about electronics you will probably not be able to be sure you are
holding a real 470 V varistor.
Novembre 19 2002
till May 5 2004