Blue Sea Systems 8410 AC Digital Meter Panel - 240V AC - Manual (Page 2)

Current Flows in a 120/240 Single Phase Three Wire System
Finally, in this diagram we add another 120V load, Load 2 connected to
L1 and Neutral. Since we have already assigned a direction to the
current
fl ow in L1 we must use the same current fl ow direction to supply
Load 2. We see that the current fl ow in L1 is equal to the sum of the
current
fl ow for Load 3 and Load 2. Just like the current in L2 was the
sum of the currents supplying Load 1 and Load 3.
The current fl ow in the Neutral deserves a special look. Since the
currents “fl ow” in opposite directions we subtract to get the net current
fl owing in the Neutral. If Load 1 and Load 2 are exactly the same size the
Neutral current will be Zero. In fact when the loads are assigned
electricians try to balance the load on Line 1 and Line 2 so that the
neither leg is overloaded, the result of this is that the Neutral current
should be about zero in a well-designed system.
Current Flows in a 120/240 Single Phase Three Wire System
(continued)
This approach requires that the 240V loads on the system be “down
stream” from the 120V loads. In other words, all of the 120V loads are
connected to the bus ahead of the 240V loads. If the bus arrangement
cannot be setup in this manner it may only be possible to monitor Line 1
and Line 2 current.
Complete Instrumentation For 120/240V Systems
To fully instrument such a system requires measuring three voltages and
three
currents:
1) Line 1 to Neutral voltage
2) Line 2 to Neutral voltage
3) Line 1 to Line 2 voltage
4) Line 1 total current
5) Line 2 total current
6)
Current fl ow in only the 240V loads
The 8410 Panel when connected as show and installed in a system with
the bus arrangement shown allows the 8247 AC Digital Meter to fully
monitor a 120/240V AC system.
It is critical that the loads be arranged as shown, otherwise the 120V and
240V load currents cannot be separated. Wired as shown, any current
required can easily be calculated. For example: If Line 1 current was 10
Amps, Line 2 current was 20 Amps, and the 240V load current was 5
Amps, then the 120V loads on Line 1 total 10A-5A=5 Amps. Similarly, the
120V loads on Line 2 equal 20A-5A=15A.
It is also possible to monitor the current fl ow to, or from, a particular load
or source, by simply passing the circuit’s hot or neutral wire through its
own
CT.
AC Voltage
There is no separate voltage sensing for the AC meters, this means that
to measure multiple voltages the power to the meter is switched from
one source to another. The meter’s internal power supply can tolerate
temporary transients but if the “off time” is too long the meter will go
through its power up process. This does not damage the meter; it is
simply a bit of an inconvenience. This should not normally occur with the
8410 Panel as the rotary switch minimizes “off time”.
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Adding a 120V Load (continued)
Based on the previous diagram we see that if we want to measure all of
the relevant currents in a 120/240V AC system we need to place our
current sensing Current Transformers (CT) in the appropriate positions.
To see the current associated with 240V loads alone, we must arrange
the distribution panel loads and buses exactly as shown. The current
measured by CT1 and CT2 measures the current fl ow total in L1 and L2.
This is the sum of the currents of the 120V loads on that line and the
current to the 240V loads. The current measured by CT3 is only that
associated with 240V AC loads.