ELECTROMAGNETIC INTERFERENCE
62305-3 (Protection Against Lightning: part 3, physical damage to structures and life hazard) and IEC 62305-4 (Protection Against Lightning: part 4, electrical and electronic systems within structures). The former specifies requirements for protecting the structure of a building from physical damage and human life from any danger using lightning protection systems; the latter provides useful information for reducing the danger of a permanent failure of electric and electronic systems due to the electromagnetic impulse of lightning. However, these provisions are not meant for preventing the malfunction of electric and electronic equipment. In most buildings, lightning rods are electrically connected to the frame; therefore, if a lighting strike occurs, the current flows to the ground through the building structure. In this situation, the lightning current flows mainly through the outer columns, but on upper floors a significant part of it also flows through the interior columns. As a result, a lightning surge may move from the building frame to electric/electronic equipment and destroy it without surge protection devices. In addition, since a magnetic field forms around a lightning current by induction, the effects of electromagnetic impulses due to lightning extend to the building inside. The level of lightning current is large, especially on higher floors near the point of strike and near outer columns and special protective measures are required in those locations.
Inverter noise General purpose inverters inevitably emit noise. As shown in Figure 2, the noise that travels via route [1] is called normal mode noise and circulates in the power supply lines. The noise that travels via routes [2] and [3] is called common mode noise and flows from the power lines to the grounding conductors and the entire building structure; it flows from the inverter in the panel through the power lines to the motor and finally to the ground via the stray capacity of the motor. Unless the motor is insulated from the steel building frame, the noise finds its way back to the inverter panel through the building frame, the ground and the grounding electrode. Common mode noise disturbs the zero
volt stability of grounding systems and by induction causes noise in cables for control and monitoring systems running near a grounding conductor or a power line, which may lead to problems such as the malfunction of communication devices, false alarms in control and monitoring facilities, faltering readings in measuring instruments and abnormal stoppage of system equipment. Communication errors in telemeters and
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Lightning protection systems safeguard the structure of a building and human life.
disturbance to diagnostic imaging displays are examples of such adverse effects of common mode noise widely experienced in medical facilities.
Lightning Figure 3 illustrates proposed protective measures against lightning. These are: l to prevent lightning currents from flowing through the building structure or to minimise the flow of currents, lighting rods and conductors must be provided exclusively for lighting currents and insulated from the building frame. When these lightning conductors are far enough away from the electrical cabling inside the building, the intrusion of surge voltage into the cabling due to induction by the lighting current is suppressed. It should be noted here that the lightning rods and conductors must be insulated from the building structure using sufficiently highly insulated materials (see Figure 3 – [4], [5] and [6])
l a grounding electrode must be provided exclusively for the lightning protection system to secure the flow of the lightning current to the ground (see Figure 3 – [7])
l electric machines (such as power distribution boards and motors), cable ladders and air conditioning ducts must also be insulated from the
building structure. For this reason, it is necessary to install them using insulating bolts and to insert insulating boards between them and the building frame (see Figures 3 – [2] and [3])
l if it is possible to use reinforcing bars (or rebars) of reinforced plastic composed of continuous fibre such as aramid fibre for building concrete instead of steel rebars, it will make the insulation and non-magnetisation of the whole building much easier (see Figure 3 – [1]).
The measures suggested make it possible to prevent lightning currents from flowing through the building structure, suppressing the intrusion of lightning surges into the steel frame or rebars of the building structure and securing an electromagnetic environment suitable for the safe operation of medical and welfare facilities inside the building.
Inverter noise As stated previously, the situation becomes complicated when the noise propagates through the building structure and therefore, like the measures against lightning, it is important to insulate the electric/electronic facilities from the building structure. Figure 4 shows a case in which rebars of reinforced plastic (for example, aramid
IFHE DIGEST 2019
©Guido Castelli
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