Cables


Magnetic field surveys are often required where new building developments are close to underground cables.  This is because, wherever there is a flow of electrical current then a magnetic field is produced.  This basic principle means that all cables will create magnetic fields when they are in use.

Most concern is often focussed on underground power cables as they carry large currents.  These cables are buried under the footpath or roadway and have a current rating from typically 100 to 550 amps.  This variation is due to the many types of cable that are in use.  Generally, it does not matter what voltage the cables are working at.  A cable working at 230 volts can produce the same magnetic field as one operating at 11,000 volts if the current is the same.

High voltages are used to increase the efficiency of the distribution system by reducing the need for large electrical currents to be transported around a cable network.  Reducing the current reduces the heating and voltage drop effects caused by the current.  This is why you always find substations near to homes.  Depending upon the load, cable systems operating at 230 volts suffer from their voltage dropping below acceptable levels after a few hundred metres from the substation.  It is unusual to find substations operating at full load.  It is more common to find that substations are built to maintain the voltage supplied to the customer.

A modern substation supplying houses would typically have a transformer that changes the voltage from 11,000 to 400/230 volts.  While the transformer reduces the voltage it increases the current.  If this substation was operating at full load, then the typical current in the high voltage cable to the substation would be approximately 37 amps while the total current in the lower voltage cables would be more than 1050 amps.  In this situation the lower voltage cables will produce the higher magnetic fields.

At even higher voltages there are some other differences.  For some cables operating at 33,000 volts, and almost all the cables that operate above this voltage level, it is normal practice for there to be 3 individual cables grouped as one cable.  This is sometimes referred to as a phase separated cable system.  The cables are normally laid with a gap between them.  This is done to increase the current carrying capacity of the cables by increasing the cooling effect of the soil around each cable.  The size of the gap varies but anything from 200 to 1000mm can be expected.  The gap however causes greater magnetic fields to be produced than would have resulted from them being laid together.  Also, using three separate cables in this way requires a broad strip of land to accommodate them, so it is normal for them to be located under the roadway in urban areas.

The magnetic field at 1 metre above the road surface over a 400,000 volt National Grid cable could reach 40 to 50 microtesla depending upon the load current.

Typical values of magnetic field found 1 metre above the footpath in a city centre can be between 0.05 and 8 microtesla depending on the position, depth, number and the loading of the cables.

Other cables and systems can produce similar magnetic fields to the power cables above.  A loose terminal screw behind a 13 amp wall socket that is part of the normal domestic wiring can cause surprisingly large magnetic field effects.  A property that was measured for comparison purposes with another home was found to create a magnetic field of 6 microtesla when the electric kettle was switched on.  This effect indicated there was a problem with the 'ring main' circuit and the owner was advised to employ an electrician to investigate the problem.

Similarly, two-way lighting circuits have the ability to create large magnetic fields.  This is because it is usual for the 'live' and 'neutral' conductors to be separately wired.  The magnetic field found near to a group of light switches in one office block was found to be greater than 13 microtesla.

Cables used to construct 'under floor' heating systems also create magnetic fields that range from about 0.5 to 20 microtesla at desk height.  The higher value could cause interference to any modern computer monitor.



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