An IT power supply system is an unearthed system where all live parts are insulated from the ground, or one point of the system is grounded through large impedance. Non-live parts of the wiring system are grounded (see CSN 33 2000-1 ed. 2). An insulated system increases the operational reliability and safety of people within the system. These systems are used in metallurgy, engineering, shipbuilding, traction systems, vehicles and hospital facilities.
The first fault
The advantage of the insulated system is the fact that the devices connected to the system may operate continuously even in case of a first fault (so-called earth fault). After the first fault, the phase voltage of the undamaged phase (or phases in three phase system) will increase to the value of the joint voltage. This must be taken into account when rating the equipment. If the earthing of non-live parts of the electrical equipment is done properly, the system is safe because when the ground fault occurs the loss of voltage is not greater than the safe voltage. The first fault must be removed as soon as possible. The relevant responsible person must be informed about this fault. When the first fault occurs, the IT system may only be operated for the necessary time, and then the fault has to be removed.
To signal the status of the insulated system, devices monitoring the insulation status or residual current monitoring relays are
used. These devices indicate a drop in the insulation status below the pre-set value. When an earth fault occurs, there is a short circuit in a different phase, which is then shut down by the surge-protecting elements. Using voltage surge protectors in large, insulated systems is necessary to make sure that they reliably protect all devices connected to this system.
One of the first elements used at the first stage is the lightning current conductor, called the spark gap, on the secondary side of the insulation transformer. The spark gap, as a non-linear element used in lightning current protectors, offers a long reaction time and high conducting capability. Its advantage when used in IT systems is that it has zero leakage current (terminals are galvanically separated). Therefore, there is no need to worry about unwanted and undesirable reactions of the insulation monitoring device.
Another element used in the first, second and third stage is the varistor, which offers fast reaction time and thanks to great technical progress achieved by manufacturers it also offers great conducting capability. However, when a large number of varistor surge protectors is installed in IT systems, it may cause problems resulting from surge leakage current. The sum of surge leakage currents through multiple protectors may reach the triggering value of the insulation monitoring device used in the system. Therefore, it is necessary to install isolating spark gap in series with the varistor (or with the varistor section in a threephase system), because the isolating spark gap behaves, in its idle state, as an insulator with a high ohmic resistance and with a long life.
Combined lightning surge and over-voltage protector
HAKEL, spol. s r. o., has recently introduced a new product for 3 x 500 V IT systems, widely used in the heavy industry area in Czech Republic, to the market. The product is marked HLSA 21-600 / 3 + 1 M S IT – combined lightning surge and over voltage protector of type 1 + 2. It is a varistor surge protector in combination with a powerful light gap connected in series; thanks to this connection there are no undesired faults in these systems caused by leakage current. The protector is installed at the interface between LPZ zones 0-1 and higher (according to IEC 62305 and CSN EN 62305 ed. 2), where it handles equipotential bonding and lighting current as well as the switching over-voltage which is created in the power supply systems entering into the given object. One of the other great advantages of this protector is the option to install it in 3 x 500 V IT systems with a central conductor – in this case, a CP / N terminal block may be used. It is important to keep in mind that this protector is produced with exchange modules, so the operator may exchange individual cartridges very quickly. Of course there is also optical and remote signalling.
The technical data sheet and more detailed information is available at: www.hakel.cz
Daniel Anděl, HAKEL spol. s r. o.