Page 220 - 35_DS702_E_2014_Lightning_Protection_Guide

such a degree that the current actually flowing is noticeably tion for the SPDs between L and N for 5 s. The SPD needs to
smaller than the possible short-circuit current at the place withstand this TOV.
of installation. In addition to this also a TOV test with √3 x U REF = √3 x
A high degree of follow current limitation prevents up- 255 V = 441,7 V for 120 min must be applied between L
stream protection elements (e.g. fuses) from tripping due to and N to simulate a loss of neutral. A safe failure mode is
a too high mains follow current. acceptable for this test.
The follow current limitation is an important parameter for If TOVs arise as a result of earth faults in the high-voltage
the availability of the electrical installation, particularly for system, a rated time of 200 ms produces U TOV = 1200 V for
spark-gap-based SPDs with a low voltage protection level. the N-PE path in TT systems.
This is also described in the German VDN guideline 2004-08: IEC 60364-5-53 (HD 60364-5-534) requires that SPDs in-
“Überspannungs-Schutzeinrichtungen Typ 1. Richtlinie stalled in low-voltage consumer’s installations have a
für den Einsatz von Überspannungs-Schutzeinrichtungen TOV withstand capability. The devices of the Red/Line
(ÜSE) Typ 1 (bisher Anforderungsklasse B) in Hauptstrom- family must be rated for TOVs according to IEC 61643-11
versorgungssystemen.“ [Surge Protective Devices Type 1 – (EN 61643-11) and meet the requirements of IEC 60364-5-53
Guideline for the use of surge protective devices (SPDs) Type 1 (HD 60364-5-534).
in main power supply systems].
Modern follow current limiting spark gaps even ensure se-
lectivity with respect to low-rated fuses in the range from
20 to 35 AgK/gG. 8.1.2 Use of SPDs in various systems
Personal protection measures always take priority over surge
Coordination protection measures. Since both measures are directly linked
In order to ensure that the various SPDs function selec- to the type of system and thus to the use of surge protective
tively, energy coordination of the individual SPDs is abso- devices (SPDs), TN, TT and IT systems and the many ways in
lutely essential. The basic principle of energy coordination which SPDs can be used in these systems will be described
is characterised by the fact that each protection stage in the following. Electric currents flowing through the hu-
must only discharge the amount of interference energy man body can have serious consequences. Therefore, protec-
which the SPD is designed for. If higher interference ener- tion measures which prevent hazardous electric shock are
gies occur, the protection stage upstream of the SPD, e.g. required in every electrical installation. Parts which are en-
type 1 SPD, must discharge the impulse current and re- ergised during normal operation must be insulated, covered,
lieve the downstream protective devices. Such coordina- sheathed or arranged to prevent that they are touched if
tion must take into account all possible interference such this could result in hazardous electric shock. This protection
as switching overvoltages, partial lightning currents, etc. measure is termed “protection against electric shock under
Coordination must be proven by the manufacturer ac- normal conditions”. Moreover, a hazard caused by electric
cording to IEC 62305-4 (EN 62305-4), IEC 60364-5-53 shock must be prevented if the voltage is transferred to the
(HD 60364-5-534) and the German VDN guideline. metal enclosure (body of a piece of electrical equipment)
When using devices from different manufacturers, proper due to a fault, e.g. a faulty insulation. This protection against
coordination cannot be assumed without making specific
calculations or performing a laboratory test as outlined in hazards which, in the event of a fault, may result from
the German bulletin 19 published by the ABB (Committee touching bodies or extraneous conductive parts is termed
for Lightning Protection and Lightning Research of the VDE). “protection against electric shock under fault conditions”.
The devices of the Red/Line family are harmonised and test-
ed with reference to their energy coordination. The limit for the permanently permissible touch voltage U L
typically is 50 V for a.c. voltages and 120 V for d.c. volt-
TOV ages.
TOV (Temporary OverVoltage) is the term used to describe In circuits containing socket outlets and in circuits contain-
temporary power-frequency surges which can arise as a re- ing class I mobile equipment, which is continuously handheld
sult of faults in medium and low-voltage systems. In case during operation, higher touch voltages, which can arise in
of TN systems as well as the L-N path in TT systems, a rated the event of a fault, must be automatically disconnected
time of 5 seconds produces U TOV = 1.45 x U 0 , where U 0 rep- within 0.4 s. In all other circuits, higher touch voltages must
resents the nominal a.c. voltage of the phase conductors be automatically disconnected within 5 s. These times apply
to earth. In case of 230/400 V systems, a TOV U TOV = 1.32 x to TN systems. In case of other system configurations, the dis-
U REF =1.32 x 255 V = 336.6 V must be taken into considera- connection times may differ.

www.dehn-international.com LIGHTNING PROTECTION GUIDE 219