DOC. MIE91093 Rev. 1.34
Page 133 of 145
In both instances, the relay behaves in a way that is similar to
an over-current relay, unless for the fact that there is no
threshold to be found, but only to measure the time delay as a
function of the current, that is related to the temperature by
the computation formulas.
The important thing to take into account with these relays is
that any current flow is taken into account; so, the test
procedure is the following:
. Adjust the test current to the desired value;
. Stop the current flow;
.
Remove the auxiliary supply
, so that the memory is
erased;
. Connect the auxiliary supply;
. Start the timing test, taking care to
correct the current
value during the test
: because of copper heating, the
current tends to decrease.
. For another test, repeat the procedure for the memory erase.
At the end of the tests, you have the time versus current
curve, from which you can derive the time versus temperature
curve, given the conversion coefficients.
For the test conduction, please refer to paragraph 1.1, test of
over-current relays.
1.18
L
OW
-
VOLTAGE AND
M
EDIUM VOLTAGE CIRCUIT BREAKER
TESTS
1.18.1. Introduction
We refer to those CB, used on Low Voltage or Medium Voltage
lines, that directly interrupt the voltage when the intervention
criteria is met.
Usually, CB’s behave as over-current relays, with high currents
(and also, usually, long intervention times). The difference
with respect to over-current relays is that CB’s
don’t have a
trip contact that drives the line opening
: they open the
line by themselves. Therefore, the criteria for timer stop must
be the fact that
the current does not flow any more
.
