Resistance of copper cables

There are often problems related to the bonding resistance between the earth reference bar (ERB) and installed devices. This is common where a device is installed using normal house wiring cable, although it will also occur whenever the installer fails to take into account the actual length of run, which may be different to the apparent length of run shown on the circuit diagram.

Many medical devices draw only a small amount of current, so that the installer may be tempted to use cable having a low cross sectional area, even though the length of cable run may be quite significant. In a medical installation the cable resistance is particularly important.

The bonding resistance from the ERB to the earth terminal of the medical device should be less than 100 mΩ. The table below shows that if a 1.0 mm2 earth conductor is used, the maximum length of run would be only about 5.5 m, although the current carrying capacity of the cable is 15 A, sufficient to supply a single 13 A socket.

The cable commonly used to connect standard 13 A sockets is 2.5 mm2 flat-twin-and-earth, which has a current carrying capacity of 27 A. However, the cross sectional area of the earth conductor of normal 2.5 flat-twin-and-earth is usually only 1.0 mm2, or in higher grade cables, may be 1.5 mm2. This means that the maximum length of run may be only 5.5 m for a radial, or 11 m for a ring, assuming that both ends of the cable are connected to the ERB.

In order to get an earth conductor of 2.5 mm2, you would need to use 6 mm2 flat-twin-and-earth cable. A 10.0 mm2 flat-twin-and-earth cable will usually have a 4.0 mm2 earth conductor, and a 16 mm2 flat-twin-and-earth cable will have a 6 mm2 earth conductor. In general other types of three core cable will have the same cross sectional area for each of the conductors.

When planning a medical installation both the resistance and current carrying capacity of all cabling must be taken into account.

Table – Resistance per meter for copper conductors