Page 131 - EPR Catalog 2015

Insulation monitoring relays for unearthed supply systems
Insulation monitoring in IT systems

In electricity supply systems, an earthing system defines the electrial potential of the conductors relative to that of 2
the earth’s conductive surface. The choice of earthing system has implications for the safety and electromagnetic
compatibility of the power supply. Note that regulations for earthing (grounding) systems vary considerably among
different countries.

The international standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT
and IT.
The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):
T: direct connection of a point with earth (Latin: terra)
I: no point is connected with earth (insulation), except perhaps via a high impendance
The second letter indicates the connection between earth and the electrical device being supplied:
T: direct connection of a point with earth
N: direct connection to neutral at the origin of installation,

which is connected to the earth

IT supply systems
The IT system is supplied either by an isolation transformer or a voltage source, such as battery or a generator.
In this system no active conductor is directly connected to earth potential. The advantage of this is that only a small fault cur-
rent can flow in case of an insulation fault. This current is essentially caused by the leakage capacitance of the system.
The fuse of the system or MCB does not respond, thus maintaining the voltage supply and therefore operation even in case of
a phase-to-earth fault.

The high reliability of an IT system is guaranteed thanks to continous insulation monitoring.
The insulation monitoring device recognizes insulation faults as they develop, and immediately reports that the value has fallen
below the minimum. This prevents operational interruptions caused by a second more severe insulation fault.

L1
L2
L3
N
PE

A1 11 21 S1 S2 S3

CM-IWN.x

12 14 L+ V V1+ V1-

22 24 L- K w A2

M Consumer
3~

2CDC 110 004 C0210 | Catalog Electronic Products and Relays 2015 | ABB 2/64