VAMP Ltd
Feeder and motor manager
Technical description
VAMP 257
VM257.EN002
VAMP 24h support phone : +358 (0)20 753 3264
51
Measured and recorded values of residual voltage
protection stages:
U
0
>, U
0
>>, (59N)
Parameter Value Unit Description
Measured
value
Uo>, Uo>>
V
Residual voltage Uo as
primary value
SCntr
Start counter (Start) reading
TCntr
Trip counter (Trip) reading
Flt
%Uo The max. fault value
Recorded
values
EDly
%
Elapsed time as compared to
the set operating time; 100% =
tripping
2.3.15.
Thermal overload protection (49)
The thermal overload function protects the line or protective
object against thermal overload. The measuring is based on the
RMS (Root Mean Square) value of the phase currents from
which the heating of the cable to be protected is calculated.
Thermal stress can be supervised by means of a thermal image.
The thermal image can be calculated from the standard heating
expression according to IEC 60255-8:
2
2
2
2
)
(
ln
n
p
I
k
I
I
I
t
⋅
−
−
⋅
=
τ
where,
τ
=
heating time constant (cooling)
ln = natural
logarithm
I = measured phase current (the max. value of three phase
currents)
Ip = preload current (corresponds to the heating level
reached)
k
=
thermal overload factor (setting value of T>)
k x I
n
= steady-state current which corresponds to the
setting value of T> (the thermal trip)
I
n
= rated
current
The heating time constant (tau [
τ
]) and the load current factor
(k) corresponding to the maximum thermal load are settable.
The factor k defines the load current value which, when
exceeded, results in a thermal trip.
The cooling time constant of the thermal overload protection is
the same as the heating time constant. Cooling time constant is
available only in motor protection mode.
The thermal overload stage is provided with a separately
settable alarm function, the setting range of which is from 60 to
99% of the thermal trip level.