Specifications

Demand Meters/Demand Meter Relays
Mechanical Indicators
76
Time interval and indications of demand meters
The time interval (t0) of a demand meter refers to the time
required for an indicated value (I
0) to indicate 95% of a fixed
input (I) when the input (I) is supplied continuously.
Additionally, for 100% of the input (I) to be indicated, a time
of approximately 3 times the time interval (t
0) is required.
Fig. 1
0.95I
I
Indicated value (
I
0)
Time tTime interval (to)
For mechanical demand meters or demand meter relays, a
coiled bimetal is heated by the input current and the thermal
change of the bimetal is used for the indication (bimetal). The
indicated value is the effective value of the input.
For electronic demand meters or demand meter relays, the
same characteristics are realized via computation using a
microcomputer.
100
80
60
40
20
0 10203040
Indicator indication (%)
Time (min)
Time interval: 2min
Time interval: 15min
Indicated value of demand meters
As described above, demand meters operate over a
comparatively long time, and unlike ordinary indicators, they
are not directly influenced by factors such as short-time
input fluctuations and flicker. Demand meters basically
indicate the average value (of demand) for load fluctuation
within the time interval.
The maximum and minimum value marker needles record
the maximum and or minimum values indicated by the
driving needle, enabling the past maximum demand values
to be obtained.
Min. value
marker needle
Max. value
marker needle
Electronic Mechanical
6
90
0
5
6
7
8
9
3
kV
LEAD LAG
%
高調波 サーマル
NSTRN
N Hz
Hz
2
1
3
ﾃ ﾏﾝﾄ
最 小 大
cos
次
分 秒
%
var
VA
NTRN
Hz
2
1
3
ﾃ ﾏﾝﾄ
最 小 大
cos
var
VA
NRS RN
Hz
2
1
3
ﾃ ﾏﾝﾄ
最 小 大
cos
10000
varh
h
VA
設定 警報
漏電
下限
通信
上限
Transition of
indicated
value of a
demand meter
Transition of
input of a
demand meter
Input (I)
t1 t2
Time interval
Time
Time
Indicated value (I
0
)
t2
P0
Selecting the time interval of demand meters
The time interval of a demand meter is selected according to the facility equipment to be monitored and the purpose of monitoring.
Example 1 Monitoring overload of a voltage
transformer
Example 2 Monitoring voltage variation of a
low-voltage bus line
Example 3 Monitoring voltage variation of a
high-voltage circuit
(Demand meter selection example)
A
10 to
30min
10 to
30min
A
V V
V
2min