WARNING! WARNING! WARNING! (was [Elecraft] ESD recommendation)

[Ian White GM3SEK]

David Woolley wrote:
>In IEE terms, the PME system is TN-C-S.  The older system is TN-S. 
>There is a rare configuation (TT), used for rural overhead supplies, 
>where each house has its own earth electrode, but these are not 
>connected to the neutral, which is only earthed at the sub-station.

TT is closest to the US system, but "rare" is a misleading word. Of 
course the large majority of UK homes are in urban and suburban areas, 
where the other two systems predominate. But if you happen to *be* in a 
rural area, TT is everywhere.

> For TT systems, RCDs are mandatory.

Yes, because an individual home ground may not be low-resistance enough 
to trip the circuit breakers on a short. A typical UK TT system has a 
100mA RCD on the whole installation, but the power circuits (wall 
outlets etc) are protected by a more sensitive 30mA RCD. This is to 
allow the power RCD to trip without disconnecting the house lights.


Returning to the topic of ESD, my Weller TCP is provided with a long 
fly-lead and a crocodile clip for equipment grounding. That lead 
connects to the ground pin of the 3-pole 24V AC plug, which is in turn 
connected to the mains ground. The clip is attached either to the 
equipment chassis or, when working on an isolated board, to the board's 
own circuit ground. The iron tip and the item being soldered are then 
bonded together.

They are also both bonded with a low resistance to the mains ground, but 
that is OK. Seems to me that the series resistors are meant for any 
*other* large items that could pick up a significant static charge - 
mainly the human operator and the bench mat.

A circuit board is not physically big enough to pick up a significant 
charge in its own right. The main hazard to circuit boards is when a big 
static charge is made to pass *through* the vulnerable parts on the 
board. There are three main scenarios where this can happen:

1. If the board is already grounded and the iron tip is not - the 
fly-lead prevents that one; or

2. If the board is already grounded, and an ungrounded operator touches 
a vulnerable pin (well, don't do that  - be grounded first, through a 
resistor wrist strap, and always pick up boards by their edges); or

3. When the ungrounded operator picks up the board, and then grounds 
himself *through* some vulnerable part of the board (well, don't do that 
either - be grounded first, through a resistor wrist strap; and make 
sure the equipment is grounded too).



-- 

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek