Many types of electronic equipment (such as computers, photocopiers and discharge luminaires) and other equipment (such as heating elements) can cause currents in the protective conductors of final circuits and distribution circuits of an installation in normal service. This article explains how these protective conductor currents are caused and how they can be a hazard to users of an installation if the current is sufficiently high and an open-circuit fault develops in a protective conductor.
A
n article in a future issue of Connections will give guidance on the requirements of Regulation Group 543.7 of BS 7671 for protective earthing for the installation of equipment having high protective conductor currents.
Causes of protective conductor current Electronic equipment Electronic equipment such as fluorescent and other discharge luminaires, variable speed motor drives and the switched-mode
TECHNICAL
Causes of protective conductor currents, and the dangers they introduce
power supplies (SMPS) of computers and other office equipment often incorporate filters to: • suppress high-frequency noise being superimposed on the mains supply by the equipment, and/or • protect a SMPS from damage due to mains-borne voltage spikes (transient overvoltages) produced by lightning strikes or the operation of network switching devices. The filters include capacitors connected between the live (line and neutral) conductors and the circuit protective conductor. In normal service, these capacitors draw a constant current, which flows in the circuit protective conductor (cpc) of the final circuit and any distribution circuit supplying the equipment. Figure 1 shows an example of this where capacitors intended to provide a path for currents caused by voltage spikes also provide a path for protective conductor current in normal service. Heating elements Some types of heating element can absorb moisture when cold. When the element is energised, the moisture provides an electrically conductive path for leakage current to exposed-conductive-parts, such as a metallic enclosure of the element. This results in protective conductor current. As the element heats up, the magnitude of protective conductor current tends to fall and stabilise at a value that depends on the nature and condition of the element.
Voltage spike
L
Capacitive filters to Earth
Load current
N
Protective conductor current
E
Fig 1 – Simplified drawing showing protective conductor current due to capacitors in the filter within an item of equipment
www.niceic.com
p59-60 Conductor currents.4.indd 59
Potential danger in the event of an open-circuit fault in a protective conductor In the event of an open-circuit fault in a cpc that carries protective conductor current in normal service, a voltage with respect to Earth will appear on any exposed-conductive-parts that were previously earthed via the cpc but are no longer earthed. If a person simultaneously touches such an exposed-conductive-part and Earth, a current will flow through the person’s body. Figure 2 shows an example of this situation. The person will be
>
NICEIC Connections Autumn 2010 59
23/9/10 12:42:39