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Home / Products / TM25t – high-frequency earth resistance meter
The TM25t high-frequency earth resistance meter
Measure the earth resistance of a transmission tower without disconnecting the shield wire. The TM25t measures at 25 kHz, so the neighbouring towers count far less than in a low-frequency measurement.
Measuring one tower while the line stays connected
The towers of a transmission line are connected to each other by the shield wire. Measure with an ordinary earth tester at low frequency and you measure all those towers in parallel, so the result says little about the one tower you are interested in. For a clean reading, the shield wire has to come off: a lot of work, and at height.
The TM25t solves this with a test frequency of 25 kHz. At that frequency each span of shield wire has an impedance of several tens of ohms, so most of the test current flows through the earthing of the tower you are measuring. The instrument compensates the inductive part automatically with a built-in capacitor bank (up to 4.2 µF), and you read the resistive part. The better the tower earthing, the smaller the influence of the neighbouring towers.
The reading tells you how well the tower can discharge a lightning current, which is what a tower earth is for.
How you measure
The TM25t uses the fall-of-potential method: a current probe and a potential probe in the ground, the test lead on the tower foot. The instrument sets the test current (20 mA) and the compensation itself. You need one instrument and one measurement per tower, not one per tower leg. Auxiliary probes may be up to 2 kΩ, so you still get a reading in dry or stony ground.
Applications
- Periodic checks of tower earthing on high-voltage and transmission lines
- Check measurement after a lightning strike or after groundwork near the tower
- Commissioning measurement of new or modified tower earthing
- Recording the results per tower (printer, memory, software)
TM25t or EM4058?
The TM25t is made for towers with a shield wire. For general earth measurements (earth electrodes, installations, buildings) and for measuring soil resistivity with the Wenner method, choose the EM4058 earth resistance meter. It measures at several frequencies (270-1470 Hz). The TM25t has no Wenner function.
Not sure which meter fits your application? Book a consultation and we will look at your installation together.
Key features
- Test frequency 25 kHz, made for transmission towers
- Measure without disconnecting the shield wire
- Automatic compensation of the inductive part
- Automatic test current of 20 mA
- Measuring range 0-300 Ω, accuracy ± 2.5 % of reading ± 1 digit
- Built-in thermal printer and memory for 2,000 measurements
- USB and Bluetooth: control and reporting with MegaLogg 3 (PC) and the BlueLogg app (Android)
- CAT IV 300 V, IP65 (lid closed), LiFePO₄ battery 12 V / 6,000 mAh, approx. 4.9 kg
Accessories
- 8x steel auxiliary probe 50 cm, copper coated
- 6x 2 m lead for connecting extra auxiliary probes
- Probe extractor
- Shielded cables 70 m and 50 m
- Lead to current probe 30 m
- Leads to potential probe 70 m and 50 m
- Cable adapter for the current electrode
- Lead to the tower (unknown electrode)
- AC adapter
- USB cable
- User manual
- MegaLogg 3 software and BlueLogg app (download)
- Carrying bag for the accessories
Also collaborate on technical challenges?
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Gijsbert from IONIO will be happy to help.
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