Q5.1. How do I obtain a good earth for a building constructed on rock?

There is no simple or easy solution to this question. Local soil conditions will have a significant bearing on the result. Guidance is given in the BS EN 62305 for structures erected on rock.

Q5.2. Is it dangerous to have a lightning conductor earth where people walk?

‘Yes’, it is potentially dangerous to have an earth electrode in a location where people walk or have access. When a lightning discharge takes place, the potential difference or gradient at ground level could be of a sufficiently high magnitude to be lethal to both humans and animals. This ‘step potential’ as it is known, can be minimized burying the earth electrode so that its top is at least 1m below the surface and to insulate the connection between the down conductor and the earth’s electrode. Further advice is given in BS EN 62305.

Q5.3. If I cannot use salt to help me achieve a lower earth reading, is there anything on the matter that can?

There are two main conditioning agents – Bentonite and conductive cement.

Bentonite is mixed with water to form slurry – its resistivity being determined by the amount of water used. It is ideal for 4 season nations.

Conductive cement (Marconite) is made with graded granular carbonaceous aggregate in place of the conventional sand or aggregate. It has a resistivity of 10ohm.cm

Both agents can be laid in a horizontal bed or poured down a drilled vertical hole, creating a lower resistance between the embedded earth electrodes and surrounding soil.

Q5.4. A minimum resistance of 10 ohms is often quoted – what action should be taken if this cannot be obtained?

A further reduction can be achieved by extending or adding to the number of existing electrodes. Or by installing a ring conductor buried a minimum of 0.6m below the ground. This ring would inter-connect the individual earth electrodes at each down conductor.

Q5.5. If I increase the diameter of my earth rod, will it significantly reduce the resistance to earth?

Increasing the diameter of an earth rod has the effect of only slightly reducing the resistance to earth. Typically increasing the diameter of a copper covered steel rod from 12.5mm diameter to 25mm diameter will increase the cost by 400%, increase the weight by 400%, but only decrease the resistance by 9.5%.

Q5.6.  What is an Earth seal?

This is a form of gland which is used to seal the earth rod when it is necessary to drive rods through the waterproof membrane under a building. It consists of a galvanized steel plate, which at its centre has a boss through which the rod is driven. This boss incorporates a gland, similar to a cable type, which is tightened round the rod to form a seal.

Q5.7. As several down conductors are needed can I bring them down in the same place?

‘No’. All standards state that they should be equally spaced, wherever possible, around the building.

Q5.8. In what circumstances can I use the reinforcing bar in concrete structures as a down conductor?

0BS-EN 62305 permits the use of reinforcing bars as a natural down conductor providing that the bars have multi-crossing points and provide a definite electrical continuity between the air and earth termination networks.

Q5.9. Can stainless steel earth rods be connected to copper earth conductor underground?

‘Yes’, provided the copper conductor below the ground has a protective cover (such as PVC) and the earth rod joint is sealed from any ingress of moisture. For example, by wrapping the complete joint with a bitumastic bandage.

Victor Oyedu, FNSE, FNIEEE, CPQ.
Power Quality and Energy Management Specialist.
Publisher at Afrienergyonline.com
CEO, FullSpectrum Energy Solutions Limited, Nigeria.