This is a story with a moral.
Have you ever thought twice about the
connectors at the rear of a computer, a Hi-Fi or for that matter
anything containing electronic circuitry?
I for one haven't.. at least not until
Don't look just yet you'll never find
your way back! but about 6 months ago I fixed a digital video
mixer and at the time the penny hadn't dropped but some time
later, when the same customer appeared with another equipment
having a similar fault, I realised what was happening (see
The latest equipment was fitted with
a row of BNC connectors and these were all fitted with phono
What's the difference?
Well when you offer up a phono lead
the centre pin makes contact first, then the outer sleeve.
When you offer up a BNC the outer case
makes contact before the inner pin.
I recall when we used to build stuff
for MoD and other knowledgable customers they were always keen
to see newly designed equipment carrying connectors with an earth
pin making first. Of course this was primarily due to their concerns
on safety rather than equipment being damaged. A young mechanical
engineer I knew rubbished this until one day when he was sprawling
across a metal rack trying to plug in a mains connector. It was
one of those famous "Plessey Plugs". All the pins were
the same length and the first pin to make contact was the mains
live one. He didn't half shout as his body made the ground return
path for the rather large capacitors in the mains filter.
Sometimes, if one gently touches exposed
metalwork on a mains powered equipment, a slight tingling or
rubbery feeling is experienced. This is usually, but not always,
confined to items having two-wire mains cables. It rarely occurs
with an equipment having a 3-wire mains lead because the rule
is that, unless special arrangements have been made, any exposed
metalwork is likely to be grounded. That is.. connected securely
to the earth pin of the mains plug. Mains wiring should have
been tested to ensure that all mains socket earth pins are properly
wired to the ground circuit and there's a maximum resistance
for this circuit, measured in fractions of an ohm, before the
ground can be deemed satisfactory. Therefore any two items with
3-pin mains plugs and 3-wire cables should have very close to
zero volts between any of their exposed metalwork, which includes
their input-output connector metal casings. Assuming two things
of course.. first that the items are properly plugged into mains
sockets and secondly that either item is not a "double-insulated"
device. I guess that the latter is designed so that a single
wiring fault cannot place any exposed metalwork at mains potential
and because of this any exposed metal can be left unconnected
to ground or floating.
Anyway, back to the zapping problem
which is the reason for writing all this. It's all very well
having a good specification for things with 3-wire cables and
3-pin connectors but what about things with 2-wire cables such
as VCRs. Safety-wise the "double-insulated" specification
should come into play and any exposed metalwork should be safe
to touch. But is it? Back to the rubbery feeling when one touches
exposed metalwork. In most instances things like VCRs when installed
in a normal environment will be grounded to mains earth. This
might be via coax cable outer braid, an aerial booster power
supply or even your chimney lashing kit. Not always though. I
have come across TV installations where everything is insulated
from mains earth and when fitting an aerial more than slight
shocks result when grasping metalwork up on the chimney. Why
is this? Usually mains input circuits carry some sort of filtering.
Not always, but sometimes, the live mains terminal will be connected
to the chassis of an equipment via a capacitor. This component
can pass alternating current from live to chassis and from there
via a finger to ground where it attempts to rejoin at the mains
supply generator or distribution system earthing arrangements.
So if we have one or a number of interconnected
equipments, all carrying two-wire mains cables there will always
potentially be a small alternating current wishing to escape
to earth. I say "potentially" because if one was to
measure the AC voltage between the 2-wire equipment chassis and
earth you would observe, on a high impedance voltmeter, up to
250 volts of potential difference. Given a loop resistance of
lots of ohms this potential difference cannot produce much in
the way of current... but the body can sense extremely small
currents hence the tingling or rubbery effect.
Unfortunately, when one looks at the
specification of some semiconductor devices, a voltage of 250
applied to a particular pin will totally destroy an input circuit.
This problem is akin to the static problem of which we are reminded
when receiving a computer part in a black plastic bag.
Back to my example. The customer said
he'd noticed a small spark between the latest equipment brought
for repair and the signal connecting lead as it was in the process
of being connected up. The odd thing was that both equipments
used 3-wire cable and normal 13-amp mains plugs with proper metal
earth pins. Clearly then it was the mains distribution system
that was at fault. Presumably the earth connection at one of
the 13-amp sockets wasn't connected.
If the original BNC sockets had been
connected to BNC plugs all would have been well. Unfortunately,
instead of the adaptors from phono to BNC being fitted to the
ends of the connecting leads, the adaptors had been left on the
equipment. In this configuration the inner pin of the phono lead
hit the socket inner before the outer metal ground connection.
The spark had jumped to the circuitry connecting to an analogue
to digital converter instead of from case to case. Result...
the AD converter had been zapped. In the repair described in
"Grumble 14" by a fortuitous chance the designer had
selected a very small wattage series terminating resistor and
it had been this that had absorbed the inrush of power rather
than the integrated circuit innards. In the latest equipment
the designer had chosen a rather higher wattage shunt device.
This ensured that the whole zapping voltage had been applied
to the integrated circuit. No less than 250 volts RMS were waiting
at the centre pin of the phono lead to be applied across a small
75ohm resistor shunting the input to the circuit whose rating
was probably no more than some 10 volts peak. The effect of the
resistor had been to shorten the time the voltage had been present
but not to reduce its immediate effect.
Not only had the main equipment been
damaged but some time earlier a very expensive graphics processing
card fitted in a computer had also been destroyed. The blame
here though was slightly different as the card was fitted with
a phono socket rather than a BNC socket. Here the problem was
not the lead but merely the faulty system earth and the fact
that the card designer hadn't taken account of zapping via this
What does one do then to avoid such
problems? Use a single mains distribution block and never use
more than one primary mains feed in a complex installation where
more than one equipment is mains powered would be a good start.
Connect everything up before plugging anything into a mains socket.
Connect everything up to the distribution block before plugging
this into the mains is a good bet and be very wary about connecting
any external wiring to your installation, because... remember
that the centre pin of that phono lead may have 250 volts on
it just waiting to zap your expensive equipment. And if the designer
has fitted a BNC connector do not (inadvertently) change this
to a phono connector.
All this is just as relevant to a modern
home or office computer system with its host of miniature jack
plugs, canon connectors, usb and ethernet plugs as the sophisticated
disco equipment described above.