LOW VOLTAGE SYSTEM EARTHING BASIC INFORMATION AND TUTORIALS

For many years the Regulations required that each l.v. system should be solidly connected to earth at only one point, that being the neutral of the source transformer. Special permission was necessary to earth at more than one point.

The Regulations also required that cables buried in the highway must have a metallic sheath. Systems earthed at only one point require the neutral conductor to be electrically separate and are now known as SNE (separate neutral and earth).

It was, and still is, the responsibility of each consumer to provide the earth connection for his own installation. This was commonly achieved by connection to a metallic pipe water main.

The growing use of PVC water mains makes this impossible for new installations and causes problems with existing ones when water mains are replaced. Gradually, supply companies developed a practice of providing consumers with an earth terminal connected to the sheath of their service cable.

This is, of course, a very satisfactory arrangement but it is not universally practical as many cables laid in the 1920s or earlier are still in use and many of these are not bonded across at joints. The arrangement is not practical on most overhead systems.

In Germany and elsewhere in Europe an earthing system known as ‘nulling’ grew up. This employed the principle of earthing the neutral at as many points as possible.

It simplified the problem of earthing in high resistance areas and by combining the sheath with the neutral conductor permitted a cheaper cable construction. These benefits were attractive and during the 1960s the official attitude in the UK gradually changed to permit and then encourage a similar system known as PME (protective multiple earthing).

Blanket approvals for the use of this system, and the required conditions to be met, were finally given to all area boards in 1974. In BS 7671 – the 16th edition of the IEE Wiring Regulations this system is classified as TN-C-S.

Providing the consumer with an earth terminal which is connected to the neutral conductor ensures that there is a low impedance path for the return of fault currents, but without additional safeguards there are possibilities of dangerous situations arising under certain circumstances.

If the neutral conductor becomes disconnected from the source of supply then the earthed metalwork in the consumer’s premises would be connected via any load to the live conductor and thus present an electric shock hazard from any metalwork not bonded to it, but which has some connection with earth. 

In order to eliminate this rare potential hazard the Secretary of State, in his official Regulations, requires that all accessible metalwork should be bonded together as specified in the IEEE Wiring Regulations and so render the consumer’s premises a ‘Faraday cage’. This is the reason for the more stringent bonding regulations associated with PME.

Under the extremely rare circumstances of a broken service neutral and intact phase conductor, there may be a danger of electric shock on the perimeter of the ‘cage’ to someone using an earthed metal appliance in a garden, even though the appliance may be protected by an RCD (residual current device) in accordance with the IEE Wiring Regulations. For the same reason metal external meter cabinets are undesirable.

In order to eliminate as far as possible the chance of a completely separated neutral, a number of precautions are taken. First, all cables must be of an approved type with a concentric neutral, either solid or stranded, of sufficient current carrying capacity.

Secondly the neutral conductor of a spur end on the system is connected to an earth electrode if more than four consumers’ installations are connected to the spur, or if the length of the spur connection from the furthest connected consumer to the distributing main exceeds 40 metres.

Where reasonably practicable, cable neutrals are joined together to form duplicate earth connections. A faulty or broken neutral will give an indication of its presence by causing supply voltages to fluctuate, which, of course, should be reported to the local DNO as soon as possible. All these measures contribute to a system which is as safe as practicable and self-monitoring.

It is the declared intention of the EI in the UK to provide earth terminals wherever required and practicable within the foreseeable future. The local DNO should be contacted regarding their requirements for the use of PME earth terminals for TN-C-S systems.

SWITCHING TRANSIENT LOADING EFFECTS ON THE SYSTEM BASIC INFORMATION AND TUTORIALS

One of the primary uses of electricity is for general lighting and the local DNO must ensure that its supply is suitable for this purpose. Repeated sudden changes in voltage of a few per cent are noticeable and are likely to cause annoyance.

The local DNO must ensure that these sudden variations are kept within acceptable levels and this means placing limits on consumers’ apparatus which demands surges of current large enough to cause lighting to flicker.

In order to evaluate flicker in measurable terms, two levels have been selected: the threshold of visibility and the threshold of annoyance. 

Both are functions of frequency of occurrence as well as voltage change.

Since both these thresholds are subjective it has been necessary to carry out experiments with various forms of lighting and panels of observers to ascertain consensus relationships between frequency of occurrence and percentage voltage change for the two thresholds.

The DNOs have used this information in setting the planning levels for flicker contained in Engineering Recommendation P28, which govern motor starting currents, etc.

The network impedance from the source to the point of common coupling between the lighting and the offending load is of paramount importance and thus the local office of the DNO should be consulted in cases where the possibility of creating an annoyance arises.

Intermittently loaded or frequently started motors, such as those on lifts, car crushers, etc., together with instantaneous water heaters, arc welders and furnaces, are all potential sources of disturbance.

Large electric furnaces present a particular problem and it is frequently necessary to connect them to a higher voltage system than is necessary to meet their load in order to achieve a lower source impedance.

Fluctuations occurring about ten times a second exhibit the maximum annoyance to most people, but even those as intermittent as one or two an hour will annoy if the step change is of sufficient magnitude.

CENELEC Standard EN61000-3-3, limits voltage fluctuation emissions from equipment rated less than or equal to 16 A and EN61000-3-11 limits emissions from equipment rated from 16 A to 75A.

UNBALANCED LOADS AND POWER FACTOR LOADING EFFECTS ON THE SYSTEM BASIC INFORMATION AND TUTORIALS

Any normal load causes a voltage drop throughout the system. This is allowed for in the design, and the cost associated with the losses incurred is recovered in the related unit sales.

Unbalanced loads

Unequal loading between the phases of the network causes an unequal displacement of the voltages. Extreme inequality causes motors and other polyphase equipment to take unequal current and perhaps become overloaded on one phase.

For this reason DNOs impose limits on the extent to which they accept unbalanced loads at any particular location in order to ensure that other consumers are not adversely affected. Installation designers need to ensure that the same problem does not arise due to an unbalanced voltage drop within the consumer’s installation itself.

While most voltage unbalance is caused by single-phase loading, the effect on a three-phase motor can best be assessed in terms of the negative phase sequence component of the voltage thereby created.

Providing that this is less than 2% the inequality of current between phases should not be more than the motor has been designed to withstand. Engineering Recommendation P29 aims to limit continuous levels of voltage unbalance to 1%.

Power factor

Many types of apparatus such as motors and fluorescent lighting also require reactive power and thereby take a higher current than is necessary to supply the true power alone. This extra current is not recorded by the kWh meter but nevertheless has to be carried by the distribution system and uses up its capacity thereby.

It also increases the losses on the system. 

A power factor of 0.7 means that the current is 1/0.7 = 1.43 times as great as absolutely necessary and thus doubles the losses (I2 R). If all the loads in the UK were permitted to have as low a power factor as this, the additional cost of the losses (if the system could stand the burden) would be in the order of £200 million per annum.