Practical Reports For Electrical Tests; Earth Resistance In Tt Systems; Earth Resistance, Voltaamperemetric Method; Creating Cables Extensions - Amprobe GP-2A Manual De Uso

10. PRACTICAL REPoRTS FoR ELECTRICAL TESTS

10.1 Earth Resistance In TT Systems

The test is aimed at checking that the RCD is coordinated with the earth resistance value. It is
not possible to assume an earth resistance value as reference limit (for example 20 as per art. 326
of DPR 547/55) when controlling the test result, while it is necessary to check every time that the
co-ordination complies with the requirements of the Standards.
The parts to be checked are represented by the whole earth installation under working
conditions. The check is to be effected without disconnecting the earth rods.
The earth resistance value measured shall meet the following relation R
R = resistance of the earth installation whose value can be set with the following
A
measurements:
• Three-wire earth resistance with volt ampere method
• Two-wire earth resistance with volt ampere method
• Phase to earth fault loop impedance (*)
• Two-wire earth resistance in the socket with volt ampere method (**)
• Earth resistance obtained by the measurement of contact voltage Ut (**)
• Earth resistance obtained by the tripping time test of the RCDs (A, AC),RCD S (A, AC) (**)
Ia = tripping current in 5s of the automatic RCD; rated tripping current of the RCD (in case of
RCD S 2 I∆n) in ampere
50 = safety limit voltage (reduced down to 25V in special environments)
(*) If the installation is protected by an RCD the measurement shall be effected upstream or
downstream the RCD short-circuiting it to avoid its tripping
(**) This method, even though not presently provided for by standards, provide values, which
compared with numberless reference 3-wire tests resulted to be reliable for earth resistance
Example
Let's assume an installation protected by an RCD Ia = 30 mA. The earth resistance is measured
using one of the methods quoted above. To evaluate whether the installation resistance is
complying with the standards in force multiply the result by 0.03A (30 mA). If the result is lower
than 50V (or 25V for special environments) the installation can be considered as coordinated as
it meets the above said relation.
In case of 30 mA RCDs (most civil installations) the maximum earth resistance allowed is 50 V/
0.03=1666Ω permitting to use even simplified methods which though do not provide extremely
accurate values, give values approximate enough to calculate the coordination.

10.2 Earth Resistance, Voltaamperemetric Method

10.2.1 Creating cables extensions

If the length of the supplied cables isn't suitable for the plant under test, You can create your
own extensions without influencing the instrument's accuracy.
For your own safety and to avoid damaging the instrument you are recommended to respect the
following indications:
a. Always use cable characterized by Insulation voltage and Insulation class complying to
Nominal voltage and measurement category (Overvoltage) of the plant under test.
b. Always use terminal connectors characterized by measurement category (Overvoltage) and
Nominal voltage complying to Nominal voltage of the plant under test (see Par. )1.4.
24
< 50 / Ia where:
A