1. What Is a Metering Junction Box? (Test Terminal Block)
A metering junction box is also called atest terminal blockortest switch, installed between the CT secondary circuit and the kWh meter. Its functions: (1) short the CT secondary side when testing or replacing the meter with the circuit energized (the CT secondary must never be open-circuited); (2) carry the CT secondary current to the kWh meter.
2. Three-Phase Three-Wire Metering Wiring (2 CTs · 2 Elements)
Three-phase three-wire systems use 2 current transformers(V/V connection), with 2 secondary winding sets connected through the test terminal block to the kWh meter:
| Terminal | Connect |
|---|---|
| CT1 secondary K1 | test block terminal 1 → meter I1 |
| CT1 secondary K2 | test block terminal 3 → meter I2 |
| CT2 secondary K1 | test block terminal 5 → meter I3 |
| CT2 secondary K2 | test block terminal 7 → meter I4 |
3. Three-Phase Four-Wire Metering Wiring (3 CTs · 3 Elements)
Three-phase four-wire systems use 3 current transformers(Y/y connection), 3 sets of secondary windings wired to the meter through a test terminal block:
| Terminal | Connect |
|---|---|
| TA1 secondary | test block 1-2 → meter I1 |
| TA2 secondary | test block 3-4 → meter I2 |
| TA3 secondary | test block 5-6 → meter I3 |
| TA common | test block 7 → meter I4 (N terminal) |
4. Test terminal block internal structure (shorting link)
Inside the test terminal block there is ashorting link (connecting bar): in normal operation the link is open (the CT secondary current passes through to the meter); during testing,close the link, shorting the CT secondary, so the meter can be removed safely.The CT secondary must never be left open— an open circuit produces high voltage that breaks down insulation.
5. Common metering wiring errors (3 red lines)
- Open CT secondary→ high voltage → insulation breakdown → safety hazard; always short the secondary before removing the meter.
- Reversed polarity→ K1/K2 reversed → meter runs backward → metering error.
- Voltage circuit miswired→ voltage phase sequence does not match current phase sequence → metering deviation.
FAQ FAQ
What is the purpose of a metering terminal block?
It provides ashortable connection pointbetween the current transformers and the meter, so the meter can be tested or replaced without shutting off power; the CT secondary must never be left open.
Why must the CT secondary never be left open?
With an open circuit the secondary current is zero, the core saturates, and ahigh voltage (up to several thousand volts)is produced, breaking down insulation and endangering personnel.
What is the difference between three-phase three-wire and three-phase four-wire metering?
Three-phase three-wire uses 2 CTs (V/v connection); three-phase four-wire uses 3 CTs (Y/y connection). The terminal block has a different number of terminals.
How do you operate the links when testing a meter?
First close the links to short the CT secondary → remove the meter → after reinstalling, open the links to restore normal metering.
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