Mains Power

Virtually all computer systems are powered from a supply provided by the local electricity supply utility.

Supply voltage

The Nominal supply voltage in the UK has for a long time been 240 Volts +/-6% however EEC legislation is changing this to 230V +/- 10% with an intermediate period at 230V +10% -6%!

These changes are being done in pursuit of the Euro Enthusiasts goal of a fully integrated and standardized Europe. In practise the changes only effect a vary limited range of current consumer items. Such as Light bulbs, Cookers, and Electric fires. Where the filaments are connected direct to the supply, and a change of supply voltage affects their power output and life expectancy.

Most "Brown Goods" {your TV, Video, High-fi, or computer} these days tend to use "Switch mode" power supplies and accept a wide range of input voltages. Typically either switch selectable between Europe/USA or fully Auto-ranging across a wide voltage range often as wide as 90V to 260V at 50/60Hz

UK Supply types

There are a number of ways of connecting a property to the mains supply. the actual one used for any particular customer depends in general on the location in the country and the length of time the supply has been installed.

If the supply is to a commercial property and the company maintain wire diagrams and test records as recommended by the IEE they should be able to identify their supply type and the last test impedance of any earth spikes.

Three common examples follow

Overhead PME

Supply cables are run overhead on poles. The customer has one or more Earth Spikes located on their property.

Most often seen in the older parts of towns or small villages.

Warning with this setup there is often a link between earth and neutral located near the meter. Without this link you need an earth spike impedance of well under 8ohms if it is to pass enough fault current to blow a 30A ring main fuse.

If this link exists an test with an Earth loop impedance tester will check for faults in the buildings wiring however it offers no guidance as to how well the ground spikes are working. A separate test of the ground spikes is required to determine this.

Under Ground PME

Supply cables are plastic/kevlar armored The customer has one or more Earth Spikes located on their property.

Warning with this setup there is often a link between earth and neutral located near the meter. Without this link you need an earth spike impedance of well under 8ohms if it is to pass enough fault current to blow a 30A ring main fuse.

If this link exists an test with an Earth loop impedance tester will check for faults in the buildings wiring however it offers no guidance as to how well the ground spikes are working. A separate test of the ground spikes is required to determine this.

Under Ground Bonded Earth

Supply cables are wire armored The safety earth for the customer is provided via the armor from a large metal plate buried underneath the substation.

This setup usually results in a very low earth loop impedance, and often gives good diversion of locally created spikes or surges to earth.


It should be born in mind that the earthing on a building was installed as safety feature. It is intended to provide a safe path for fault current originating at the live on the supply transformer back to that supply transformer, for a period long enough to blow the supply fuse.

If the ground in which an earth spike is situated dries out, or is contaminated, with nonconducting fluids such as diesel the spike will not work as well. Also if the ground is excessively acidic or alkaline resulting in corrosion of the spike this will adversely affect its performance.

Where any doubt exists as to the suitable of a buildings earth wiring for dumping RFI, spikes, static discharges, etc. It is often easier to look at isolation or ferro-resonant transformers that make lesser demands on the high frequency performance of the buildings earthing.

Mains Supply and distribution problems


No mains supply for a period of more than 2 cycles {1/25th of a second}.

This has to be the most familiar problem. But whether the cause was you plugging in one too many appliances and "blowing" a fuse, or a fault on the distribution network. The effect is the same when a computer loses it's power supply any data not written to disk will be lost.

Also the sudden loss of power while a disk drive is actually writing to a drive and disk heads are therefore positioned over the drive surface may damage the actual drive.


Supply voltage is reduced to below the lower nominal supply voltage for an extended period. This problem can be caused by supply network but is more commonly caused by changes in load patterns in the immediate vicinity.

Where the supply is on an overhead pole run, it is not unusual to see the supply voltage drop by one or two Volts per cable span when a heavy load such as a cooker or electric shower are switched on.

On industrial estates or light industry areas, high current manufacturing equipment such as furnaces, ovens, or thermal lamination units are more likely to be the cause.

Effect on computer systems depends on how far the voltage actually drops and what the actual computer load is at the time in comparison to the rated capacity of the power supply.

As the voltage drops the power supply responds by trying to draw more Current to compensate. If the computer is a fully expanded machine running at close to the rating for the power supply this increase in current may either damage the power supply to cause it to shut down. If the power supply shuts down see the problems associated with blackouts.

If the power supply keeps running but provides a reduced voltage to memory or disk subsystems. The problem may manifest as disk or memory errors.

Power sag

Supply voltage is reduced to below the lower nominal supply voltage for a short period. This class of problem is caused by changes in the supply network. But it is far more likely to be the start up load of a piece of equipment at the site affected.

Favorite candidates in the transport industry are, Air compressors, Arc welders, Refrigerated containers, Chiller units for refrigerated storage, lift motors and electric gantry cranes.

Most modern computer systems will tolerate occasional short sags lasting a cycle or two. Beyond this the same problems as for brownouts may manifest them selves.

Power surge/over voltage

Supply voltage rises above the upper nominal voltage for an extended period.

This is the opposite of a sag/brownout, and is most commonly cased by the removal of a major load from the network. Where the load on a section of the power grid changes, the grid substations may compensate by changing the transformer taps after a few cycles.

But where it is a neigbouring factory shutting down for a summer break, or bank holiday weekend. There is far less chance of any automatic correction being applied.

Where the nominal voltage is exceeded there is a chance either of damage to the switching transistor in the power supply or of the power supply shutting down.


Sudden increases in supply voltage typicality lasting 100uS to 10,000uS {a half cycle} often up to 6,000 Volts.

Caused by lightning strikes to high voltage grid pylons, or network switching.

Local causes are switching inductive loads {Electric motors} and static discharge.

Mains Transients

Sudden increases in supply voltage typically lasting up to 100uS and typically rising to as much as 20,000 Volts.

Caused by lightning strikes to nearby grid pylons, or substations.

Local causes are switching inductive loads {Electric motors} and static discharge.

Earth transients

As for Mains Transients but voltage is between Neutral and Earth Rather that Live and neutral.

This problem usually shows up as corrupt characters on printouts, characters appearing on screen which have not been entered. Other wise unexplained network timeouts and re-transmissions.

More extreme cases show up as blown serial driver chips in terminals, printers, computers and modems.

RFI line noise

Radio frequency interference.

A couple of miles of mains distribution cable makes a very good arial!

Fortunately for computer users considering the ever growing number of radio control systems, radio car alarms, mobile phones, etc. Switch mode power supplies are fairly good at blocking this problem.

however where serial {rs232} cables or network cables are run in close proximity to mains cables it has a nasty habit of coupling across and entering the system by the back door so to speak.

This problem usually shows up as corrupt characters on printouts, characters appearing on screen which have not been entered. Other wise unexplained network timeouts and re-transmissions.

Frequency Variation

Changes in supply frequency, of more than +/- 2.5%

This is very unusual for the UK national grid network, it is normally maintained in a much tighter range. If you monitor the frequency it is very unusual to see it outside the range 49.95Hz to 50.05Hz.

How ever this is not true of many standby generator systems.

Small variations in supply frequency are not a problem for standalone computer systems. However larger changes especially drops in frequency can cause the same symptoms as Sags.

Can be more of a problem in process control applications especially if some of the equipment is mains synchronous and some not.

Harmonic Noise

This is where the mains supply voltage is overlayed with one or more secondary voltage wave forms at a multiple of the supply frequency.

This most commonly 3rd or 5th harmonics which for the UK 50Hz supply are at 150Hz and 250Hz.

These harmonics are in fact caused by loads with a high crest factor such as the switch mode power supplies found in modern computers.

While not a problem for the computer they present problems to the supply network as they increase losses in supply cables and transformers.

Current supply meters do not measure these harmonics. How ever it is perfectly possible that the next generation of meters may.

Residual Current Circuit Breaker RCCB

These are a really great safety invention. They operate by comparing the current the current flowing in the Live and Neutral wires if these differ by more than a predetermined amount usually 30mA or 100mA depending on application they cut the power off.

From a safety point of view this is great, however if the office has been fitted with a single unit covering lights and all power points. Which is a ridiculously common setup. You have a very good chance that when the janitor changes that dud light bulb that he/she thought was turned off. It will save them a sore set of fingers and blackout the whole office.

Where an office has a number of PC's, Fax machines, photocopiers, etc. That may have been fitted with varistor based surge protector networks. the possibility exists that when when you experience a surge or spike the surge protectors will collectively divert enough energy to the earth wiring to trip the more sensitive of these devices.

Earth leakage Circuit Breaker ELCB

Forerunner of the RCCB. Measures current in the earth conductor.

For problems see RCCB.

Costs and benifits of power conditioning

Applying any form of power conditioning to the power supply used for computer equipment can be thought of in much the same terms as taking out an insurance policy.

The level of "Insurance" that it is worth taking out obviously depends on the "Value" of the item to be insured. In the same way that the main and fairly new family car would probably be best insured "fully comprehencive" and an older less valuble and less frequently used 2nd car "Third Party". Not all items of computer equipment will justify the same level of protection.

Typical office PC

Running wordprosessing and spreadsheet. The cost to the company of the PC stopping is around 1-2 hours of the users time to run scandisk and re enter any "lost" data. Where the disk is corrupted beyond the abilities of scandisk to repair this can rapidly rise to 8 hours or more as they re-install windows, restore from their last backup, and recreate any work lost.


The costs to the company of having a prinetr unavailable due to mains problems or a powwer cut depends heavily on what it is used for.

If it is the only printer or used to print time critical documents such as Drivers job sheets, POD's, or picking lists. The printer would be regarded mission critical with a sustantial business cost if it is unavailable.

Where however there are plenty of other printers available providing a filtered power supply is advantageous there may not be a cost justification in supplying it from a standby power source in the event of a power cut.


For most busnesses no phones means no customers and no money. So it easy to justify the additional costs of installing either a PABX with its own standby batteries, or adding it to the list of critical devices to be supplied from the standby supply provided for other computer equipment.


Where a computer runs as a server sharing time critical business data between a number of users. The business costs resulting from a loss of power will always justify the costs of providing protection for supply disterbances and standby power for at leased long enough to print off a printout of the current days work and perform a gracefull shutdown of the server.

In many cases the costs of a standby generator to provide power for an extended period. Hours or days rather than minutes may be justified especialy in rural areas where finding and fixing faults in long distribution runs and often bad weather can take a substantial time.

I'm a gambler what are the odds

For the most destructive lightning damage the odds depend on the preceding weather. The wetter the ground and in consequence the better its conductivity, the higher the proportion of the strike energy that goes streight down in to the ground, and the smaller the destructive area of the strike.

The more connections there are to a computer the more vonerable it is, so multi-user systems with a number of connections to screens and printers around a building are far more likly to suffer damage that the single user PC's/Printer combination that is pluged into a single mains socket.

From experience with our customers. The most destructive single thunder storm I know of followed 6 weeks of nice dry summer weather. It destroyed beyond repair more than 5% of the unprotected multi-user computer systems in its path.

Given our more typical soggy weather most of the time the liklyhood of catastophic damage is much lower. rough figures would be 0.02% for any storm, 20 storms per year, giving odds of 1 in 250 for the system being destroyed in any one year.

The real snag is not the cost of the computer, in most cases that will be covered by insurance. It is however the consequential losses that result from a computer crash. This is at leased 10 times more likely than the destruction of the computer. Typical estimates for consequential losses would be 50% of turnover for period affected typicaly 1 to 5 days and, 2,500 per MB for reentering lossed data.

Sample power setups

This section attempts to show a representative sample of the setups that are possible when looking at useing computer systems or computer networks for busness critical applications.

The ideal office

If designing the ideal office for a computer system. First find a site in a valley where the ground is damp enough to provide a good earth but no risk of flooding. A few nice tall trees above you on the sides of the valley, to minimize lightning risks to your new site.

Now if we can have a nice little housing estate for neighbours with no awkward or heavy power loads.

The electrical equipment that we wish to protect consists of our PABX which has it's own batteries, and therefore does not need to be connected to a standby supply but would benifit from being included in the list of equipment to be provided with a filtered supply.

Kit requiering standby supply
Item VA Qty Total
Server 500 1 500
Console 100 1 100
System printer 150 1 150
Total     750
Kit requiering filtered supply
Item VA Qty Total
Standby UPS 850 1 850
PABX 1000 1 1000
Printers {noncritical} 150 2 300
Terminals {noncritical} 75 8 600
Total     2,750

To impliment this setup we first split the incoming supply through separate RCCB's for safety. and then pass the computer and PABX supply trough a filter, which can be a simple isolating transformer as with our choice of neighbours we do not have a problem with sags or surges.

And finally we use a relatively unsophisticated Standby UPS or Line interactive UPS to protect our critical computer equipment against the occasional power cut.

The ideal office PME feed

Where the supply is on a PME rather than bonded earth basis. It may be advantagious to provid a seperate earth point for the computer equipment. This linkes in at the isolating transformer.

Less than ideal location

It is frequently no posible to pick ones nabours or as in many cases with the transport industry, sags surges and other mains problems are caused by other equipment esential to operations. {Air compressors, Arc welding, Electric crains, etc.}

If this is suspected or shown by a mains test the preceding setup can be modified by substituting a CVT for the isolating transformer in the previous example.

Integrated Units

Where the office is an existing building and the existing layout does not make installing a centralized filted supply practical. Or where the majority of the load is deamed to be critical. It may make sence to use 1 or more of the smaller integrated units to provide protection on an office by office basis.

Single PC{Home/small office}

In the home/small office environment the typical setup is to use an integrated unit incorporating a level of filtering sutable for the local supply, and standby power for 15 to 30 minutes.


Manufacturers Links

Standards documents

There are a number of standards documents that describe power problems, testing for them, and design practise. If you are really interested the family of standards documents IEC801-nn subsequently renumbered as IEC60801-nn and available from the British Standards institute as BSEN60801-nn are of relevance as are BSEN61000-n-nn. these are typically available at between 50 and a 100 pounds a document.