Cable Test Voltage in MV Switchgear (Uct, IEC 62271-200)

Short answer

If a switchgear panel allows testing the cables connected to it, the manufacturer must declare one or more rated cable test voltages, Uct, AC or DC. They are the maximum voltages that can be applied to the cable with the panel connected. The standard verifies them with a type test and asks the user to specify them with margin.

Short answer

To test a medium-voltage cable at high voltage, it is usually disconnected from the switchgear. That takes time and forces opening terminations. IEC 62271-200, 2011 edition, provides another option: that the panel itself is designed to withstand the test with the cable connected. In that case, the manufacturer must assign one or more rated cable test voltages, Uct, under clause 4.102. The standard introduced this optional characteristic and its tests in the 2011 edition. For the high-potential test on medium-voltage switchgear and cables, see hi-pot testing of medium-voltage switchgear and cables.

What the standard defines

Uct (AC). Rated power-frequency cable test voltage. It is the maximum AC test voltage that can be applied to the cables while they are connected to the panel and the panel may be in service (clause 4.102.2).
Uct (DC). Rated direct-current cable test voltage. It is the maximum DC test voltage that can be applied to the cables connected to the panel, which may be in service (clause 4.102.3).

A note to clause 4.102.3 indicates that a DC value may be considered valid also for very low frequency tests (for example, 0.1 Hz).

The key word is “maximum”. Uct is not the voltage that must be applied to the cable: it is the ceiling the panel withstands. The test value that corresponds to the cable is defined by the cable specification or the test procedure, and must stay below the Uct of the panel.

What the panel must withstand

Clause 5.105 explains the design provision. The panel can be designed so that the test is made from a dedicated test connection or from the cable terminations. In that case it must withstand the rated cable test voltage applied to the parts that remain connected to the cable, at the same time as the rated voltage is applied to the parts of the main circuit designed to stay live during the test.

In other words: while a test set applies high voltage to the cable, the busbar can stay energized, and the panel must tolerate that combination.

The type test

Clause 6.2.101 defines a dielectric type test for the cable test circuits. It applies only to panels with one or more assigned Uct values. For each value the following are applied simultaneously:

The rated voltage Ur, as a single-phase voltage between earth and all the phase conductors on the busbar side, connected together.
The voltage Uct, AC or DC, to each pole of the cable test connection, one by one, with the other connections earthed.

The duration is 1 minute for AC Uct and 15 minutes for DC Uct. If both voltages are AC and of the same frequency, their polarities must be opposite. If there is segregation between the cable test connections and the busbars, the busbar-side voltage can be omitted.

How Uct gets onto the panel rating plate

The manufacturer designs the panel to test connected cables
The panel is built with a test connection or suitable terminations (5.105)
The manufacturer assigns AC and/or DC Uct (4.102)
Type test with Ur on the busbars and Uct on the connection (6.2.101)
The Uct is declared for the user

What the user must specify

Clause 8.107 asks the user to specify Uct values with an adequate margin over the cable test voltages expected to be applied. That expected voltage comes from the cable to be tested and the test method, whether direct current, very low frequency or power frequency.

In the enquiry and the order, the standard includes the rated cable test voltages in the list of data supplied (clause 9.1, item k). Whoever does not specify them may receive a panel without that characteristic.

The tests of the panel at the factory and after assembly are explained in switchgear tests at the factory and after assembly. The cable test is detailed in VLF cable testing.

What to check before testing a connected cable

That the panel plate or data sheet declares a Uct of the type to be applied (AC or DC).
That the cable test voltage stays below that Uct, with margin.
That the parts of the main circuit that remain connected during the test are those the manufacturer foresaw. Uct does not cover any configuration.
That the test is made through the test connection or the terminations the manufacturer indicates.

For the cable test procedure, see the guides on on-site withstand voltage testing and on VLF cable testing. This guide only explains the value of the panel.

What not to do

Do not apply to the cable a voltage higher than the Uct of the panel with the cable connected.
Do not assume every panel allows testing with the cable connected: it is an optional characteristic.
Do not confuse Uct with the rated voltage of the panel or with the insulation level: they are different values.
Do not apply the test from a point other than the test connection or the terminations the manufacturer provides.

When to call us

At Tiza Group we test medium-voltage cables at high voltage and verify with you that the test respects the limits of your switchgear. Learn about our medium-voltage switchgear testing service. Write to us and we will quote it. Field testing is done in Colombia, or elsewhere on request.

References

IEC 62271-200, High-voltage switchgear and controlgear - Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV, 2011 edition, clauses 4.102 (4.102.1 to 4.102.3), 5.105, 6.2.101, 8.107 and 9.1.

Frequently asked questions

What is Uct in a medium-voltage switchgear panel?

It is the rated cable test voltage. For alternating current, it is the maximum power-frequency test voltage that can be applied to the cables while they are connected to the panel, which may be in service. For direct current, it is the maximum DC test voltage. IEC 62271-200 defines them in clause 4.102.

Does the DC test voltage also apply to VLF?

The standard notes that a rated DC cable test voltage may be considered to apply also to very low frequency tests, for example at 0.1 Hz. That equivalence is a note of the standard, not a requirement.

Do all panels come with a Uct?

No. It is an optional characteristic. The manufacturer assigns it only if the panel is designed to allow dielectric testing of the cables connected to it. Without it, there is no declared cable test voltage.