BOOT CAMP 193 (20/09/01)

ELECTRICAL SAFETY

According to Department of Trade and Industry statistics for accidents in the home (http://www.dti.gov.uk/homesafetynetwork/dy_rrcd.htm) on average each year around 2000 people receive hospital treatment for electric shock and sadly 25 people die as a result of electrocution. Faulty electrical appliances are also responsible for an estimated 5000 fires, a further 500 hospital casualties and 20 deaths; clearly electricity can be dangerous stuff!

A letter from electrical engineer and dotcom reader Harry Leeming prompted us to review the advice we've been giving concerning delving into the backs of desktop PCs to carry out upgrades or repairs. It's something I and many others do almost every day without thinking but for anyone unfamiliar with mains powered devices this can be a hazardous venture, so we'll begin right away by saying that if you do not feel competent or have the slightest doubts about your abilities to undertake work on your PC leave it alone and let the professionals sort it out.

That said, if you take the proper precautions, and you know which end of a screwdriver to hold, there should be no risk to your personal safety but if you're determined or sufficiently ham-fisted you can still damage your PC…

Rule number one therefore is always UNPLUG YOUR PC from the mains before you do any work on it. In the past we've suggested that it can be left plugged in but with the mains socket switched off, the idea being that the case remains earthed, which will dispel any static charges that may have built up on your clothes or body. Static electricity can destroy delicate microchips and we'll have a closer look at that in a moment, but back to mains safety.

Switching the mains off but leaving the plug connected has been common practice for many years and several PC and peripheral manufacturers still carry that advice in instruction manuals and on their web sites. However, we've decided to play safe and advise against it from now on as there is a small but very real chance that although the mains on your wall socket is switched off the wiring inside the PC could still be 'live'.

This can happen if the live and neutral connections to a wall socket or the house wiring are reversed. There's also no guarantee that the earth connection does anything either. As Harry Leeming points out he has encountered wrongly wired sockets and even entire houses on several occasions – he says that early on in his career he found that up to 20% of household sockets were wrongly wired! If you are at all concerned about the safety of the wiring in your home or office you should have it checked by a qualified electrician, you can also buy simple plug in socket testers from most electrical suppliers for around £10 to £15 or have a look at: http://www.kewtechnik.co.uk/socket.htm

If your wiring or sockets are wired back to front and you were to remove the lid of your PC with it still connected but switched off, the only place where you could inadvertently come into contact with 'raw' mains is on or around on/off switch; fortunately the connections are usually well insulated. However the possibility exists and in a worst-case scenario you could touch an exposed mains connection with one hand whilst the other is touching the earthed metal case, creating a path for the current to run across your chest, via your heart! Mains shocks can be very unpleasant but as can be seen from the statistics they are not usually fatal, nevertheless we don't want to loose any readers, so UNPLUG IT FIRST!

As a matter of interest the mains connection from the switch usually go straight into an earthed metal case box called the power supply unit (PSU). This converts the mains into the low voltage and harmless DC supplies needed by the PC. For the record most PSUs provide outputs of 3.3, 5 and 12volts, the first two are used by the motherboard and other electronic components whilst the 12 volt supply is used to power motors in disc drives and cooling fans. Do not be tempted to dismantle a computer PSU, you could compromise its electrical safeguards and devices called capacitors store an electric charge that can still give you a very healthy 'jolt' several hours after the PSU has been disconnected from the mains!

Finally a few word on static precautions. CMOS microchips can be damaged by electro-static discharge (ESD), admittedly it is rare and modern devices are quite well protected but once again it's not worth risking it. Static charges of several thousand volts can easily accumulate on your body or clothing, especially if they contain a high proportion of man-made fibres; some types of carpet and floor covering can also generate large static charges and if you frequently receive electric shocks when touching metal objects you should be doubly careful before fiddling around inside your PC or handling static-sensitive devices.

There are two ways of getting rid of static build up, the quick way and the proper way. Simply touching an earthed metal object like a radiator or water pipe will do the trick but you risk getting a mild shock, and if the pipe or radiator isn't right next to the PC there's a good chance another charge will build up as soon as you move around. The correct way to dispel a static charge is to use an earthing strap. These come in many shapes and styles but the simplest, quickest and for most users the most effective type is a wrist strap. These have a connecting wire, fitted with a high-value resistor (typically 2 to 4 million ohms) that attaches by plug or crocodile clip to an earth point or, at a pinch, the metalwork of your PC. Earthing wrist straps are not expensive, prices start at around £3 and you can also get special mains plugs with safe earthing points; both are available from computer accessory suppliers and electronic specialists like Maplin (www.maplin.co.uk).

Next week – Audio recording on your PC

JARGON FILTER

CMOS

Complimentary Metal Oxide Silicon – family of microchips, susceptible to damage from static discharge, extensive used in PCs and other digital electronic devices

OHM

Unit of electrical resistance

SOCKET TESTER

Device that plugs into a normal mains socket and checks the status of the wiring

TOP TIP

This tip is for all Internet nosey parkers and trivia fans. If you've every wondered what's going on behind the scenes at your favourite web site a visit to www.netcraft.com will reveal all. Simply type in the address of the site you are interested in and click Search, Netcraft then carries out an in-depth survey of the workings of the site and will report back a few seconds later with information about the server and its operating system, and if available, details of when it was installed, how long it has been running, the operator, its numeric IP address and any other little titbits it can uncover.