No fear of stray current: eight questions for the expert

Stray current can occur in any electrical installation. Put simply, these are currents that take an unplanned path outside the intended circuit – in other words, flowing through the ground instead of through the cable. Such occurrences give rise to concerns. The issue is a recurring topic, particularly for livestock farms.

BKW, which has 125 years of experience with stray current and earthing concepts, takes these concerns seriously – while at the same time offering reassurance: “Stray current has existed for as long as humanity has used electricity as an energy source. With correct handling and regular checks, there is no cause for concern,” says Philippe Rothermann, Head of Grid Operations Planning & Engineering at BKW. In this blog article, the expert answers the eight most pressing questions.

What is stray current and how does it occur?

In everyday language, stray current is also referred to as leakage current or vagabond current. These terms imply current flows that appear to creep aimlessly and freely through the ground like a vagabond. In fact, with all electrical installations it can happen that small amounts of current find their way back into the electricity grid outside the circuit created for this purpose. The larger a building is and the more extensive and distributed the electrical installations within it are, the more likely it is that current may flow away through the ground instead of through the cable intended for this purpose. The quantities of these stray currents are very small and pose no danger to people in the building. This is because every electrical installation is legally required to have an earthing system that ensures any stray current flows safely and in an orderly manner back into the electricity grid (see also the questions: “What do the solutions look like?” and “What is the correct procedure in the event of stray current?”).

Where does the problem with stray current lie?

Stray current can hardly be completely prevented. However, if electrical installations are maintained in accordance with regulations and serviced regularly, stray current does not cause any problems. Building owners must ensure that electrical installations are neither outdated nor poorly executed and that the cables have good insulation. In addition, they are legally required to have regular inspections carried out (see also the question: “What rules and laws are there?”).

What do the solutions look like?

The most important solution for dealing with stray current works – to explain it figuratively – like a vacuum cleaner that sucks up the stray current along the electrical installations and then returns it to the power grid in an orderly way. This vacuum cleaner is called earthing. Put very simply, earthing consists of a piece of metal that works in a similar way to a lightning conductor. Physics explains the phenomenon using the principle of least resistance, according to which current always flows exactly where there is the least resistance. With good earthing, the piece of metal in question offers the stray current in the ground such a path of least resistance.

What is the correct procedure for dealing with stray current?

Every homeowner, operator of an animal husbandry business or owner of a building is responsible for ensuring that the electrical installations are operated safely and properly. Among other things, to minimise the risk of stray currents, electrical installations must be inspected regularly by a certified electrical inspector. The requirements for dealing with stray current depend heavily on the use of a building or facility. Rooms with water, such as bathrooms, are significantly more susceptible than dry living areas. Large buildings with many people, such as hospitals, require more extensive safety precautions than smaller structures such as a tool shed. The situation is particularly challenging in animal husbandry businesses: here, animals often stand on wet, conductive floors and at the same time come into contact with metal equipment such as drinking troughs or milking machines, which can intensify the effects of stray current. Regular and professional inspection of the electrical installations is therefore essential. In the event of unusual electrical occurrences, visible damage to the building installation or uncertainty, an electrician or electrical inspector should be consulted as a precaution even before the next regular inspection.

What rules and laws are there?

There are a great many laws and standards – and information sheets for virtually every device and every type of cable. In this jungle of regulations, realistically, an overview can only be maintained with the help of an expert. It is therefore important that installation work is carried out only by qualified specialists. In addition, building owners are obliged to have their electrical installations inspected regularly by qualified specialists. The frequency of these inspections depends on the use of the building and the respective hazard potential. As the distribution system operator, BKW is obliged to ask all owners to submit proof of safety once the inspection period has expired.

What expertise does BKW have in dealing with stray current?

For 125 years, BKW has built, operated and maintained its own transformer stations and substations. During this time, the company has developed extensive know-how and earthing concepts. BKW knows the solutions and standards and applies them without compromise. It can also carry out special measurements for earthing systems itself. For its customers’ domestic installations, BKW works with the Association of Swiss Electrical Inspectors (VSEK). This association looks after the electrical installations of households or industrial facilities and has specialists who approve the installations for operation and carry out inspections. Within this association, there are trained experts for stray currents in agriculture.

How badly are BKW’s underground power lines affected by stray current?

The underground power lines that are built and operated by grid operators such as BKW are largely protected against stray current. This is because underground power lines – unlike electrical installations in buildings – have a metal sheath that encases them. This sheath ensures that the cables of the power lines do not emit any stray current into the surrounding environment, as the metal acts like an earth.

Who monitors BKW?

For every installation that BKW builds, it must submit all planning specifications to the Federal Inspectorate for Heavy Current Installations (ESTI). This step is a mandatory part of the planning approval procedure, as the building permit procedure for electrical installations is called. For each of our installations, ESTI checks whether it complies with all laws and ordinances. Before BKW is allowed to put an installation into operation, it must be approved by ESTI. In addition, there is a statutory requirement to remeasure the earthing systems every ten years.

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