Where To Connect ESD Wrist Strap?

[LoveOC]

I used to connect it to a metal surface which is touching the ground or the house heating radiator,
Is that wrong?

[shaithis]

Anything thats grounded really. A radiator was always my first choice, or make sure the PC is plugged in (but turned off) and clip onto the chassis.

I think the last time I used one was back in the late 90s though!

[DanceswithUnix]

I used to plug mine into one of these:



with the other pair of connections going to anti static mats to make sure the whole bench was a static safe area.

[peterb]

The important bit is the 1Mohm limiting resistor, which may be in the plug (like the one above) or built into the wristband lead itself (or both, it doesn't matter). This will limit the current flow should you inadvertently touch a live component while you are earthed.

Of course you shoukdn't be working on ESDs in a live environment!

But like Shaithis, I rarely use an ESD strap for computer assembly, which is really little more than assembling reconstructed modules, where the individual components are internally connected.

It IS worth touching an earth point when inserting the CPU, and also avoid wearing clothing that is prone to generating static - most synthetic fabrics, silk etc and avoid touching connectors on peripheral cards, but in general, PC components are designed with a reasonable degree of protection built in.

[g8ina]

My ground is ground, a 6 ft steel rod driven well into the area around the base of the house, it doubles as the ground for my ham radio antennae.

When I say ground I mean *ground*

[chinf]

The important bit is the 1Mohm limiting resistor, which may be in the plug (like the one above) or built into the wristband lead itself (or both, it doesn't matter). This will limit the current flow should you inadvertently touch a live component while you are earthed.

This current limiting resistor also will helpfully protect the electronic component itself should there be any static charge on it which gets conducted to earth via you.

To the OP: the key is to prevent high currents - so in practice you don't have to ensure that everything is at real earth potential, only that everything is at a similar potential. This means that it's fine to attach your wrist strap to a PC chassis (that's not truly grounded) as long as you stage your components via an antistatic mat that's also attached to it. The antistatic mats have an inherent higher resistance (but still slightly conductive) that will safely bring your components to the same potential as you and the chassis.

I do still use a wrist strap and mat, and I use a grounding plug similar to the one in DanceswithUnix's post. Mine has two 4mm sockets and one stud, which is more convenient for grounding cables that often terminate in banana plugs rather than press studs. Using these precautions is a minor inconvenience, but preferable to finding that a piece of hardware is acting up and then wondering if it was static damaged.

[peterb]

Chinf, it isn't high current that causes static damage, it's high voltage that punches through the thin semiconducting layers in the devices. The 1Mohm resistor allows any static charge to dissipate safely while protecting the wearer from a low impedance power source.

[chinf]

Voltage is electromotive force; it doesn't directly in of itself cause damage. It can however (which is why it is sometimes called "electrical potential") cause a large current to flow, which *is* the direct cause of damage to sensitive devices (or us).

In other words - voltage doesn't involve any movement of electrons (think: static charge), current *is* the movement of electrons, and it's this movement which (with sufficiently large intensity) can cause damage through local heating or electromigration.

The resistance provides protection by ensuring there is at least 1Mohm resistance when bridging two objects (or an object and ground) at differing electrical potentials; this minimum resistance prevents damage by limiting the resulting current flow to a non-damaging level. Of course, by bridging the object to ground you also dissipate static charge away which you also want.

[peterb]

I do know the difference! But a semiconductor junction that may be capable of carrying (say) 100mA at 5 volts could be damaged by a high voltage static discharge even though the current may be a fraction of 100mA. Static build up is a high impedance source, i.e., high voltage low current.

The purpose of the 1Mohm resistor in the earthing lead of the wrist strap is to protect the wearer against contact with an intrinsically low impedance source, while still providing a safe and continuous ground path to prevent static build up.

If you have ever removed a wooden jumper worn over a synthetic material, you will have heard the sparks as the instantaneous voltage may be several Kv, but the current is tiny - but that voltage will break down thin semi-conductor junctions. Field effect devices are the most prone to damage because of the high field strength associated with static build up.

That said, if a field effect or other ESD device is built into a circuit, the surrounding components usually provide a sufficiently low impedance path to prevent damage.

In a conventional semiconductor junction, over voltage will cause the junction to break down, and the resulting current will cause damage unless it is limited (zener diode regulators rely on that effect) but they are less likely to be affected by static because the current available from static build up in a commercial or domestic environment is small. In a fixedly effect device, that static discharge will disrupt the junction because it is so thin, so I accept your point that it is the current that does the damage, but that has nothing to do with the 1Mohm resistor which is for the safety of the user.