Lately I've become aware of increasing
numbers of equipment failures in respect of mains supply problems.
Why should this be so?
The answer partly, at least, is related to the so-called "harmonisation"
of continental and UK mains supplies.
Years ago, before the National Grid, UK mains voltage depended
on where you lived, so one are might receive 250 volts DC and
your neighbour might be getting 120 volts AC. To cater for the
variations one had to firstly ensure that an AC/DC type equipment
was purchased, if in fact you received DC mains, or if you were
on an AC supply (or DC supply) you had to ensure the voltage tapping
arrangement at the rear of the TV set, for example, was set correctly.
Gradually, over the years, DC mains disappeared from general use
but some AC/DC rated items (the name commonly associated for "DC"
equipments) remained on sale in the shops. Usually the reason
was that an AC/DC radio was cheaper to manufacture than an AC
only type.
Nowadays, nearly all equipment is for use on AC mains only.
There are now two principal types of mains driven equipment. One
uses a transformer that changes the mains voltage to something
more manageable for the components used, and at the same time
providing isolation for the user from potentially dangerous voltages.
The second type, generally considered to be more efficient, doesn't
use a standard mains transformer, but instead converts the incoming
mains supply to DC then uses a high frequency switching arrangement
to provide the voltages demanded by the circuit of the TV set,
radio or whatever. This circuit also provides isolation for the
user, but this time via a high frequency transformer.
This second type of arrangement is not particularly more efficient,
but can be made a lot cheaper and is extraordinarily light in
weight compared with its mains transformer relation, especially
when high powers are being used.
Now, to consider the harmonisation of
European mains and the unforeseen consequences that have arisen...
Under the arrangement UK mains, which was understood by everyone
to be 240volts AC suddenly became 230volts AC and continental
mains that were 220volts AC also became 230volts AC.
Did they though?
No they didn't. Because the change would have incurred untold
billions of pounds expenditure (that's thousands of millions incidentally,
the day-to-day US billion not the UK billion) actual physical
changes to the National Grid were not part of the deal.
All that happened was a statement to the effect that from today
the mains throughout Europe is 230volts and that's an end to it.
There has been a change since the declaration though. Because
the harmonisation relies on "tolerances" to make it
viable, when new mains cables are installed the contractors do
not have to adhere to the old 240volt standard. Instead they can
use less copper in their cables and supply a lower voltage.
Unfortunately this makes it slightly worse for consumers, as they
now get a bigger swing in their mains voltage depending on how
much current is being drawn down the new, thinner cables. This
isn't too bad unless you have a really old TV. That's the sort
whose picture gets smaller when the mains voltage drops (a common
problem in days of old).
Designers of electrical equipment, who used to be familiar with mains circuitry, and the old standards, seem to have been caught a little bit wrong-footed. Once these chaps retire and their old ideas retire with them, all should be well.
Some examples…. I repaired several
light units designed in Germany and being used by Hampshire Local
Authority. These light units employed a varistor (a protection
device for preventing damage from spikes and transients in mains
supplies).
The rating of the varistor was fine for 220volt German mains (re-designated
230volts) but not suitable for 240volt UK mains (re-designated
230volts).
Bearing in mind that manufacturing tolerances of say 10% can effectively
reduce the working voltage at which the varistor clamps the mains
to be lower than the actual incoming mains voltage (not spikes
and transients I might add, the actual constant RMS level), it
is not surprising that a varistor can sometimes explode.
This is because they are designed for sporadic short term excursions
not just a higher average voltage. Small devices, 14mm diameter
explode, 20mm devices burn up and the associated fuse (if indeed
a fuse is fitted , and this not always the case) will blow, and
larger devices such as 32mm types will be undamaged and just cause
the equipment fuse to blow.
Recently I found a wall-mounted "intelligent"
telephone switchboard or PABX had failed after a mains failure.
When the power supply board was removed it was discovered that
the varistor rated at 250volts was short-circuit.
Maybe the PABX was of continental design? If so the standard varistor
WOULD have been rated at 250volts because that's OK for 220volt
mains.
The UK "standard" varistor is 275volts.
All very well STATING that the mains is 230volts, but that doesn't
change the fact that it's really 240volts. Admittedly the new
and old tolerance figures make this a maximum of more than 250volts
for both, but tell that to a designer in Germany who still designs
to the 220volts standard. Cost of new varistor 30pence, new fuse
10pence, BT quote £650.
I wonder if the staff from the generating authority bother to
investigate all the claims for damaged equipment following a mains
surge? If they did then they wouldn't be so ready to cough up
for repairs following a mains failure.
A couple of other points…. Equipment using switching power supplies will continue to work normally even when some of their component parts have degraded beyond serviceability but.... cut their mains voltage and turn them on and they will not work. Ostensibly these equipments failed when the mains supply failed, BUT they had already potentially failed, maybe six months earlier.
Finally computer power supplies. These are always of the switch-mode variety and often will cater for virtually ANY mains voltage from around 100 up to 270. They include a circuit which detects whether the incoming mains is like the US type, say 115volts or European 230volts and switch automatically to the correct level. Jolly useful because there's no little setting switch needed and a common design can perform worldwide. Unfortunately, if there's a "brownout" or a period when our UK mains voltage is very low immediately before being restored the circuit which has dutifully selected 115-volts will explode when 230volts suddenly appears across its input.
I wonder how the consequences of the recent mains failure (Jan2009) in nearby Bransgore is managed. From the evidence quite a large voltage suddenly appeared at local mains sockets, and this destroyed all sorts of equipment. In one shop the PABX, one computer, the electric blind, the burglar alarm and the transformer supplying a LAN switch all failed.
How many equipments have YOU got that are permanently connected to 13-amp sockets just waiting for a similar mains problem?
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