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FROM THE BENCH

E-mail: fromthebench@gcmagazine.co.uk

Mischief in the mains Alan Bennett describes some bad aspects of the mains power supply

W

e take the 230V domestic electricity supply very much for

Surges and spikes

granted. But it can be a demon,

The nastiest things a mains supply can

Mains-borne interference

produce are short sharp voltage spikes,

Interference of various types, over a wide

based home entertainment gear and PCs.

again most harmful to electronic – as

range of frequencies, may be carried in

A while ago we had a sudden spate of

opposed to simple electrical – equipment.

mains supplies. It’s generally manifest

equipment in for repair: everything from

‘Electronic’ is anything which uses

as radiation from cables rather than

TVs to toasters, from disc players to phone

semiconductors, microprocessors or

direct injection into equipment because

chargers. Some of it smelled of burned

memories, embracing PCs, all entertainment

the latter’s internal power supply – with

varnish, some had molten bits inside. Some

gear and such appliances as washing

its large low-impedance DC reservoir

had power supply sections blown apart. We

machines, which incorporate electronic

capacitors – acts as a filter. Some powerline

were able to resuscitate a few bits of this

control systems. Spikes can punch through

data networks, even those licensed by

gear, but much of it had to be scrapped. It

and break down insulation in the primary

Ofcom, can cause interference radiation

had all come from a small residential area

sections of power supply circuits. By

from cables to spoil DAB, FM and amateur

centred on a street-corner substation. I

various routes they can find their way to

radio reception. Shielded mains cable is not

don’t know what had happened in there

control-microprocessor chips, causing them

commonly available, but cures are generally

but it had certainly churned out a lot more

to derail at best (cured by a mains reset or

found in using screened signal leads and

than 230V that day. The electricity supply

reboot) or to fail completely, necessitating

double-screened (satellite) RF cable, and

company was incredibly difficult over

replacement, at worst. Memory chips

ensuring that mains and signal feeds are

compensation and equipment replacement,

likewise can suffer from data derangement

not routed together.

giving many of their customers a bad time.

(calling for re-programming) or internal breakdown. Replacement is the only cure

Prevention and cure

for the latter, followed by weary data

Mains spikes and surges may be held at

Seldom in the UK does something as

reloading unless – for instance in a TV, set-

bay by using suppressors, most easily

dramatic as that happen. More common are

top box or disc player – a pre-programmed

found in special 13A distributor socket

power cuts, in which the supply disappears

memory chip is available. This generally

strips. Locally-generated interference is

without warning, to the detriment of the

has to be specified per model because the

best suppressed at source where possible.

operation of PCs, in which data can be

data varies between different screen sizes,

For hi-fi and home cinema perfectionists

corrupted or lost altogether: computers do

features, hardware and sometimes even

there’s a wide range of very expensive

not like being shut down in a disorderly

production runs.

mains suppression and conditioning

JUNE 2011

manner! It’s much worse in some other

GET CONNECTED

CHINE

especially to delicate microprocessor-

Outages and brownouts

32

THE DEVIL IN THE MA

The very least that spikes and the like

equipment. How about a 1.5kW electronic

countries – a friend of mine who shoots and

can do is cause momentary blips on the

(switch-mode) mains pure sine-wave

edits videos on a Greek island has to use

sound and vision, for instance freezing

regenerator at £1500? Special mains plug

an expensive UPS (Uninterruptible Power

of a Freeview TV picture and squawks or

fuses, guaranteed not to cause interference

Supply). In rural areas of the UK fed from

clicks on sound. While dirty mains power

or degrade the electricity as it passes

overhead lines strung on poles cuts are

can be due to neighbouring electricity

through, at £34.99 for three? You need to

more common than in town, usually resulting

users, e.g. industrial premises, most often

give them time to burn in, I understand:

from the weather: wind and thunderstorms.

the offenders are right inside the house:

check them out at www.amr-audio.co.uk

Lightning in particular can superimpose

thermostats in boilers, freezers and fridges,

horrific voltage spikes on overhead power

central heating time switches, light dimmers,

accessories by manufacturers and

feeds, to play havoc as described below.

washing machines and others. The use of

enthusiast magazines have been incredible.

Brownouts are reductions in supply voltage, harmless to light bulbs, kettles and toasters, but life-threatening to electronic equipment using switch-mode power supplies; they can compensate for reasonable voltage reductions but get into trouble when they run out of road, as it were.

cube-shaped multi-way mains adapters

The Advertising Standards Authority

can cause trouble; strip-type distributors

upheld complaints against Russ Andrews

are better. Any common impedance in

Accessories Ltd. regarding their claims

the mains supply (for instance contact

for mains cords and spike suppressors,

resistance, extension leads) can impose the

failing them on grounds of Substantiation,

mains consumption ripple of one piece of

Truthfulness and Comparison:

equipment on another.

see www.asa.org.uk

Some of the claims made for power

Profile for Get Connected Magazine

Get Connected: June 2011  

Get Connected: The Magazine of the Electrical Industry

Get Connected: June 2011  

Get Connected: The Magazine of the Electrical Industry