IEC 61869-6 sets the insulation of the secondary terminals of an LPIT according to the cable length to the secondary equipment: under 10 m a reduced level is enough (820 V power frequency and 1.5 kV impulse); over 10 m, 3 kV and 5 kV apply. Optical connectors have no requirement. The preferred auxiliary supply is 110 or 230 V AC and 24, 48, 110 or 220 V DC.
Short answer
A low-power instrument transformer (LPIT) delivers a small analog or digital signal to relays, meters and control units. Its electronics sit close to high-voltage circuits, and its secondary terminals connect to low-voltage equipment. IEC 61869-6, 2016 edition, says what voltage those terminals must withstand.
The value depends on one thing: the length of the cable between the LPIT and the secondary equipment. Under 10 m a reduced level is accepted. Over 10 m the general level of IEC 61869-1 applies. Optical fiber has no requirement.
What an LPIT is and where the relevant insulation sits
IEC 61869-6 defines the LPIT as an assembly of one or more current or voltage transformers. It may include transmission systems and secondary converters. Its purpose is to carry a low-power signal, analog or digital, to measuring instruments, meters, protection or control equipment.
The general diagram in the standard has these blocks:
- primary sensor, in contact with the high voltage;
- primary converter;
- transmission system, which can be an electrical cable or an optical fiber;
- secondary converter, which delivers the output signal;
- power supply on the primary side and on the secondary side.
Not every LPIT has every block. A passive LPIT, with no active electronic components, has no power supply. The standard calls an LPIT that does have them an electronic LPIT.
The insulation this guide covers is that of the secondary terminals and of the low-voltage components. It is not the high-voltage side insulation, which is governed by the general part of the series. Conventional instrument transformers are explained in the current transformer guide, and the sensor family in non-conventional instrument transformers.
What Table 601 says
Clause 5.3.5 of IEC 61869-6 adds a table to what IEC 61869-1 already says. Table 601 gives the withstand capability of the secondary terminals and of the low-voltage components according to the transmission system.
| LPIT transmission system | Power-frequency withstand voltage | Impulse withstand voltage |
|---|---|---|
| Electrical cable shorter than 10 m | 820 V | 1.5 kV, 1.2/50 µs wave |
| Electrical cable of 10 m or more | 3 kV | 5 kV, 1.2/50 µs wave |
| Optical connectors | Not applicable | Not applicable |
The 1.2/50 µs wave is the standard lightning impulse shape: it rises in 1.2 µs and falls to half its peak in 50 µs.
Why cable length changes the level
The standard gives the reason. When the total electrical cable to the secondary equipment does not exceed 10 m and the associated earthing impedance is low enough, the common-mode voltage is not expected to exceed a safe value. Common-mode voltage is the voltage that appears at the same time on all conductors of a cable with respect to earth.
In that case the insulation of the terminals and of the low-voltage components can be reduced to the level of IEC 60255-27, table C.3. That level must also meet the requirements of the equipotential bonding system, called PEB in the standard, for a working voltage of 150 V.
When the cable is longer, the common-mode voltage can grow. The requirements of IEC 61869-1 then apply, and so does the second row of the table.
The table writes the boundary as “shorter than 10 m” and “10 m or more”, while the clause text says “does not exceed 10 m” and “exceeds 10 m”. For that reason the design should avoid a cable of exactly 10 m.
- How does the signal reach the secondary equipment?
- Optical fiber: No withstand voltage requirement at the optical connector
- Electrical cable: Do the total cable and its earthing meet the conditions in the standard, with less than 10 m?
- Yes: Reduced level of Table 601 (820 V and 1.5 kV), with the equipotential bonding system for 150 V
- No, more than 10 m: General level of Table 601 (3 kV and 5 kV)
Optical fiber and transferred overvoltages
Optical connectors appear in the table as not applicable. Fiber has no electrical continuity, so it does not carry a substation overvoltage to the secondary equipment.
Clause 6.11.4 says the same in other words. The main cause of transferred overvoltages is the switching of high-voltage equipment, and the requirement does not apply if the transmission system is non-conductive, such as an optical fiber.
Auxiliary power supply
Clause 5.3.601 covers the rated auxiliary supply voltage. It is the voltage measured at the supply terminals of the device itself while it operates. It includes the auxiliary resistors or accessories that the manufacturer supplies or requires to be installed in series, but not the conductors that connect to the supply.
The preferred values are:
- in alternating current, 110 V or 230 V;
- in direct current, 24 V, 48 V, 110 V or 220 V.
The supply terminals must meet what IEC 60255-27, table C.7, defines. If your substation DC system has a different voltage, resolve that difference in the specification before buying the equipment.
LPIT partly indoor and partly outdoor
Some LPITs have one part in the yard and another in a cabinet. Clause 4.2.601 asks the manufacturer to state which part is for indoor use and which for outdoor use. That declaration matters because each part has its own ambient conditions and is tested under them.
How the insulation is checked
The withstand voltage test of the low-voltage components is in clause 7.2.601. It is done with the LPIT unpowered, new and dry, under these laboratory conditions: temperature of 10 to 30 °C, relative humidity of 45 to 75 % and pressure of 86 to 106 kPa.
Each independent circuit is tested against all the other circuits connected together and to earth. The manufacturer describes which circuits are independent. For example, the secondary converter and the merging unit can be.
- Power frequency. The voltage is nearly sinusoidal, with a frequency of 45 to 65 Hz. It is raised without appreciable transients up to the table value and held for 1 minute. There must be no breakdown or disruptive discharge.
- Impulse. The standard lightning impulse of IEC 60060-1 is used, with an output impedance of 500 Ω and an energy of 0.5 J. Three positive and three negative impulses are applied, at least 5 s apart. No disruptive discharge is accepted, and after the test the LPIT must still meet the basic accuracy tests.
These are type tests. Check that the report matches the configuration you are buying and that the installed cable respects the assumed length.
What not to do
- Do not assume that an 8 m cable and a 25 m cable require the same thing. Table 601 separates them.
- Do not use the reduced level without checking the earthing. The reduction holds only with a sufficiently low earthing impedance.
- Do not ask for a voltage test on an optical output as if it were an electrical terminal.
Related topics
Terminal insulation and auxiliary supply are only part of the requirements of an LPIT. Electromagnetic immunity, faults, reliability and temperature and accuracy with harmonics are covered in their own guides, as are rated burden and delay. The destination of the signal is the merging unit.
When to call us
At Tiza Group we review an LPIT specification before purchase: cable lengths, earthing, auxiliary supply and the way the signal reaches protection. If you will connect the digital output to your protection and control system, we offer IEC 61850 engineering. For testing the installed equipment, see our current and voltage transformer testing, and to define the specification, the engineering design and studies service. Field work is in Colombia, or elsewhere on request; specification review can be done remotely, worldwide. We quote the work according to scope.
Related guides
- Electromagnetic immunity of a low-power LPIT
- LPIT faults, reliability and temperature
- Accuracy with harmonics and noise in an LPIT
- Merging unit: IEC 61869-9, 9-2LE and testing
- Non-conventional instrument transformers (LPIT)
References
- IEC 61869-6, Instrument transformers - Part 6: Additional general requirements for low-power instrument transformers, edition 1.0 of 2016, clauses 3.1.601, 4.2.601, 5.3.5 (Table 601), 5.3.601, 6.11.4 and 7.2.601.
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition, clause 5.3.5.
- IEC 60255-27, Measuring relays and protection equipment - Part 27: Product safety requirements, tables C.3 and C.7, to which IEC 61869-6 refers.
Frequently asked questions
Why does the standard allow lower insulation when the cable is shorter than 10 m?
With a short cable and a sufficiently low earthing impedance, the common-mode voltage should not exceed a safe value. That is why IEC 61869-6 allows the insulation to be reduced to a level of IEC 60255-27, table C.3, which must also meet the equipotential bonding system requirement for a 150 V working voltage.
Does an LPIT with a fiber-optic output need an insulation test on that output?
No. Table 601 marks optical connectors as not applicable, because fiber does not conduct electricity. The other connection points of the LPIT, such as the auxiliary supply and the electrical terminals, are tested.
What auxiliary supply voltage does the standard prefer for an LPIT?
110 V or 230 V AC. 24, 48, 110 or 220 V DC. These are preferred values, not mandatory ones, and the supply terminals must meet the insulation of IEC 60255-27, table C.7.