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The following standards are to be ob-
served for the earth-termination system:
DIN 18014 Foundation earth electrode
(German), IEC 62305-3 (EN 62305-3)
and DIN VDE 0151 (German)
Material and minimum dimen-
sions of earth electrodes
with respect to corrosion

Figure 9.32.6 Example of an intermeshed earth-termination system

If the transmitter has an insulation strength of < 500 V a.c., be taken into account. According to the EC type examination
the intrinsically safe measuring circuit is earthed. In this case, certificate (PTB 99 ATEX 2092), the internal capacitances and
surge protective devices which in case of a nominal discharge inductances of BXT ML4 BD EX 24 surge protective devices
current of 10 kA (8/20 µs wave form) have a voltage protection (Figure 9.32.6) are negligible and do not have to be taken
level below the insulation strength of the earthed transmitter into account for the conditions of intrinsic safety (Table
must be used (e.g. U p (core / PG) ≤ 35 V). 9.32.2).

Type of protection – Category ia, ib or ic? Maximum values for voltage U i and current I i
The transmitter and the surge protective device are installed According to its technical data, the intrinsically safe transmitter
in Ex zone 1 so that type of protection ib is sufficient for the to be protected has a maximum supply voltage U i and a maxi-
4 … 20 mA current loop. The surge protective devices used mum short-circuit current I i when used in intrinsically safe ap-
(ia) fulfil the most stringent requirements and are thus also plications (Figure 9.32.7). The rated voltage U c of the arrester
suited for ib and ic applications. must be at least as high as the maximum open-circuit voltage
of the power supply unit. The nominal current of the arrester
Permissible maximum values for L 0 and C 0 must also be at least as high as the short-circuit current I i of
Before an intrinsically safe measuring circuit can be put into the transmitter to be expected in the event of a fault. If these
operation, it must be demonstrated that it is intrinsically safe. marginal conditions are not observed when dimensioning the
To this end, the power supply unit, the transmitter, the cables surge arresters, the surge protective device can be overloaded
and the surge protective devices must fulfil the conditions of and thus fail or the intrinsic safety of the measuring circuit is
intrinsic safety. If required, energy buffers such as the induct- no longer ensured due to an impermissible temperature rise on
ances and capacitances of the surge protective devices must the surge protective device.

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