Zap

 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 recently.

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 Grumble14)

The latest equipment was fitted with a row of BNC connectors and these were all fitted with phono adaptors.

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 route.

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.

 

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