Electromagnetic Surveys Limited

Transport

Trackside EMF surveys are sometimes requested as transport systems are not free from electric and magnetic fields (EMFs). This is because electricity is used both as a power source and as a controlling mechanism.

Trains and Trams powered from overhead wires will create electric and magnetic fields along the length of the wire. Similarly, where power is provided by electrified track, electric and magnetic fields will exist along the length of the track.

Trains

The producer of transport's largest electric and magnetic fields (EMFs), is the electric train. Electric fields from the 25,000 volt overhead wires create electric fields at platform level greater than 1,000 volts/metre. Where there are many overhead wires, for example at a major station, the electric field can peak around 4,000 volts/metre. This compares to about 1,500 volts/metre under most 132,000 volt power lines.

The level of magnetic field created depends upon the type of train.

Mainline or Intercity trains.

These trains have powered carriages at each end of the train and that is where the largest magnetic fields can be found. Only staff has access to these parts of the train. For the passengers, the magnetic fields are much lower in the carriages at about 1.6 microtesla but with higher values within the flexible corridor between the carriages. The National Radiological Protection Board (NRPB) has measured these magnetic fields and those within the Buffet car to be up to 50 microtesla.

Suburban trains.

These are mainly powered by DC motors with the DC current being either supplied directly by electrified rails or from an AC overhead line with the AC to DC conversion taking place on the train. AC magnetic field measurements show the effect of the rectification on such trains where a 100Hz magnetic field is produced.

The 100Hz field comes from the 'ripple' or residual AC current that is not completely filtered out after the rectification process. On some trains all the motor control equipment is fixed to the underside of the floor beneath the passenger compartments. The NRPB has measured the AC component of magnetic field at the floor level and in some cases found it to be up to 1,000 microtesla at a frequency of 100Hz when the train accelerates.

There are many designs of suburban trains and there is a large variation in the levels of the magnetic field produced within the passenger compartments. There is usually also an equally large DC or 'static' magnetic field on these trains.

Maglev Trains.

We have not had the opportunity to make magnetic field measurements on or near to magnetic levitation trains but the fields can be expected to be as high, if not higher, than the trains mentioned above. With magnetic levitation systems, strong magnetic fields are required to lift and power the trains.

Trams

These create magnetic fields in the same manner as trains do. However the modern trams that we have measured appear to have slightly lower magnetic fields than in the older trains mentioned above. The modern tram has an AC magnetic field of a few microteslas in the passenger compartment, above the electric motors that drive the wheels.

Motor Cars

It does not matter if the engine is petrol or diesel for it is the ancillary equipment that produces an AC magnetic field. All modern cars have alternators to charge the battery once the engine is running; however, this is not the major source of EMFs within a car.

The highest magnetic fields are usually found to be associated with the heater fan motor. Although this motor is a DC motor it has a commutator. This has the effect of creating AC magnetic fields that have a frequency that depends upon the speed of the motor. The higher the speed, the higher the frequency of the magnetic field produced. Fields greater than 6 microtesla at frequencies above 600Hz can be found in some cars when the heater is switched to full speed.

With the move to turn cars into more 'environmentally friendly' modes of transport there is a need to make cars either fully or partly electric. This has the potential to create larger magnetic fields within these cars.

Another consequence of electric cars is the need to recharge the batteries. This also has the potential to increase the ambient levels of magnetic fields we all experience around our homes as the battery charging will take place overnight. This is a time when magnetic fields in our homes are usually at their lowest values. The exceptions to this are those homes that have 'Off Peak' heating systems or where washing machines etc are timed to operated during the night to take advantage of 'White Meter' electricity tariffs.

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