What Is a Measuring and Disconnect Terminal Block? Types and Selection Criteria
A measuring and disconnect terminal block is a screw-connection DIN rail terminal that isolates a circuit and connects an instrument without removing conductors.
A measuring and disconnect terminal block is a screw-connection DIN rail terminal block whose built-in disconnect mechanism lets you isolate the line and connect a measuring instrument without removing the conductors. It is used for commissioning, testing and maintenance in the secondary circuits of current and voltage transformers and in meter, instrument and protection relay connections. A standard feed-through terminal block has no disconnect or measuring mechanism, so it cannot replace this terminal block in instrument transformer secondary circuits.
Types
| Type | What it offers | When to choose it |
|---|---|---|
| Without test probe | Disconnect mechanism only | When isolating the circuit is enough and measurement is taken elsewhere |
| Single test probe | One test connection point in addition to the disconnect | When measuring or checking from a single point |
| Double test probe | Two test connection points in addition to the disconnect | When both leads of the instrument connect through the terminal block |
Selection criteria
- Cross-section: The terminal cross-section must match the secondary conductor it takes. Because current transformer secondaries are 1 A or 5 A, the conductor cross-section rather than the rated current usually decides the choice in these circuits.
- Rated current and voltage: The 6 mm² types in the catalog are rated 41 A and the 10 mm² types 57 A; all are rated 750 V.
- Number of test probes: Choose the type without a probe, with a single or with a double test probe according to how you will connect the instrument.
- Housing color: Gray and beige carry no standard meaning; use them to tell current and voltage circuits apart in the panel according to your own grouping scheme.
Common mistakes
- Leaving a current transformer secondary open: If the secondary of a current transformer is opened while its primary carries load, a dangerously high voltage appears at the terminals and the transformer can be damaged. Short the secondary or connect the instrument in series before opening the disconnect.
- Shorting voltage transformer terminals: A voltage transformer secondary cannot withstand a short circuit; do not let the leads touch when connecting a test lead.
- Leaving the disconnect open: After testing, check that the disconnect is closed and that the meter or relay is measuring again.
- Making do with a feed-through terminal: When testing is needed later, conductors have to be removed and the risk of a wrong connection rises.
Measuring and disconnect terminal blocks in the catalog
All 7 active products in the Measuring and Disconnect Terminal Blocks category are from the Onka MRK series:
| Type | 6 mm² (41 A) | 10 mm² (57 A) |
|---|---|---|
| Without test probe | Gray, beige | — |
| Single test probe | Gray | Gray |
| Double test probe | Gray, beige | Gray |
Current transformers that feed the measuring circuit, knife disconnect terminal blocks for isolation only and 2-in 2-out test terminal blocks as check points in control circuits are in separate categories.
Frequently asked questions
What is the difference between a measuring disconnect terminal block and a test terminal block?
A measuring and disconnect terminal block offers a mechanism that isolates the circuit without removing conductors, plus a test connection. A 2-in 2-out test terminal block is a check point that joins four points; no disconnect mechanism is stated in the product data of the models in the catalog.
How is the terminal block opened in a current transformer circuit?
First short the current transformer secondary or connect the instrument in series, then open the disconnect. The secondary of a current transformer whose primary carries load is never left open.
Should I choose 6 mm² or 10 mm²?
The terminal cross-section must match the conductor. If a heavier conductor is used because the distance between the current transformer and the meter is long, a 10 mm² terminal block is needed.
When is the type without a test probe enough?
When measurement is taken at another point and the terminal block only needs to isolate the circuit.