Floating earth

Dave Plowman (News) wrote:

If left alone he'd persuade himself that it meant we could use T+E!
This is person who, when tasked with buying network cable (after having attempted to start a job with 40+m runs of HDMI and finally been persuaded into CAT5 converters when it didn't work) managed to find the most expensive one CPC did. It was the purple low-smoke CAT6 stuff and about 3 times the price of boggo CAT5.
--
Scott

Where are we going and why am I in this handbasket?
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On Wed, 10 Dec 2014 19:14:13 +0000, The Natural Philosopher

The high impedance half mains voltage effect from such emi filters when the chassis is left disconnected to earth. Not normally a problem to human touch unless several such bits of kit are all linked together without benefit of a safety earth connection.
It can be a real problem as far is computer interfaces such as serial ports are concerned, especially when one end is properly earther and the other, predictably a laptop powered from its universal mains charger, lands up with half mains voltage on its zero volt rail.
Most of the time, the D shell connector will establish a cross bonding of the zero volt rails via the shield wire before the signal pins make contact but the classic D connector wasn't designed with this specific hazard in mind and it's quite possible to have a signal pin make first contact, especially likely with a worn connector.

From a human perspective that's essentially the truth of the matter but, with computer interfaces, that's not usually true even when the input pin has low enough impedance to drop the voltage to a safe limit.
The problem is the transient discharge from the filter capacitor if the mains voltage doesn't happen to be close to the zero crossing point in its cycle at the time of contact. The discharge current, whilst extremely brief, will have ample energy if the voltage is much above the ten volt mark at the critical moment of contact.
Since it varies in magnitude between zero and 170 volts 100 times per second, the odds of the voltage being more than 10 volts are going to be in excess of 80%, at a rather conservative guesstimate. With the risk of damage being so high, it's a gamble I'd rather avoid taking, thank you very much!

True but to avoid this particular problem, you need to have the charger unplugged from the laptop's charging socket whilst connecting the serial cable. You can safely reconnect the charger afterwards (but remember to disconnect it again before you disconnect the serial cable).

"Striped"? What does "striped" mean in this context? It's not an expression I've come across before other than describing a colouring pattern in artwork or wall decorations and suchlike.
--
J B Good

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