# Relay test block

Canonical URL: https://eloragrid.ai/symbols/relay-test-block
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: TB
Drawing use: Schematic
Reference families: IEC, Manufacturer

Multi-pole test facility between CT, VT and trip wiring and a relay, allowing safe isolation and test injection.

## In plain English

### What does Relay test block mean?

A relay test block sits in the secondary wiring between CTs, VTs, trip outputs and a protection relay. Inserting a test plug or opening the block isolates the relay from the plant, automatically shorts CT circuits before they are opened and isolates trip outputs, so the relay can be tested with secondary injection while the primary circuit stays in service.

### A simple example

The schematic should show the order in which poles break and make, because CT shorting must occur before CT circuits open. The symbol alone does not show that sequence, so the terminal arrangement of each pole needs to be drawn or referenced to the manufacturer data.

### What the drawings help you do

Check CT poles short before they open. Confirm trip outputs are isolated when the block is opened. A relay test block isolates and shorts circuits in one operation; a single CT test terminal handles only one conductor.

### This particular reference

Multi-pole test facility between CT, VT and trip wiring and a relay, allowing safe isolation and test injection. A relay test block isolates and shorts circuits in one operation; a single CT test terminal handles only one conductor.

### Terms

- Coil: A wound conductor that can produce a magnetic field when energised.
- Terminal: An identified point where a conductor connects.
- Signal: Electrical information passed between devices, rather than necessarily the main power supply.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Transformer terminals for measuring and test circuits](https://www.phoenixcontact.com/en-ie/products/terminal-blocks/transformer-terminal-blocks?p=7)
- [Disconnect and knife-disconnect terminal blocks](https://www.phoenixcontact.com/en-au/products/terminal-blocks/disconnect-and-knife-disconnect-terminal-blocks)


## What it represents

A relay test block sits in the secondary wiring between CTs, VTs, trip outputs and a protection relay. Inserting a test plug or opening the block isolates the relay from the plant, automatically shorts CT circuits before they are opened and isolates trip outputs, so the relay can be tested with secondary injection while the primary circuit stays in service.
The schematic should show the order in which poles break and make, because CT shorting must occur before CT circuits open. The symbol alone does not show that sequence, so the terminal arrangement of each pole needs to be drawn or referenced to the manufacturer data.

## How to read it

- Check CT poles short before they open.
- Confirm trip outputs are isolated when the block is opened.

## Information to show or reference

- Number each pole
- Show CT shorting arrangement
- Show trip isolation poles

## Important distinction

A relay test block isolates and shorts circuits in one operation; a single CT test terminal handles only one conductor.

## Facts

- Isolates: CT, VT and trip circuits
- Key feature: CT shorting before opening
- Preview: Functional illustration of the device role; not a product footprint or certified standard graphic

## Public references

- [Transformer terminals for measuring and test circuits](https://www.phoenixcontact.com/en-ie/products/terminal-blocks/transformer-terminal-blocks?p=7) — Phoenix Contact. Public manufacturer range of transformer and test terminals for measuring and protection circuits.
- [Disconnect and knife-disconnect terminal blocks](https://www.phoenixcontact.com/en-au/products/terminal-blocks/disconnect-and-knife-disconnect-terminal-blocks) — Phoenix Contact. Public disconnect and test terminal families used in secondary circuits.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.