{"id":17,"date":"2026-11-01T14:08:36","date_gmt":"2026-11-01T14:08:36","guid":{"rendered":"https:\/\/holdvolt.com\/blog\/?p=17"},"modified":"2026-11-01T14:08:36","modified_gmt":"2026-11-01T14:08:36","slug":"sample-iec-62040-3-vfi-ss-111-explained","status":"publish","type":"post","link":"https:\/\/holdvolt.com\/blog\/sample-iec-62040-3-vfi-ss-111-explained\/","title":{"rendered":"IEC 62040-3 Explained: What VFI-SS-111 and Class 1 Mean on an Online UPS Datasheet"},"content":{"rendered":"\n

IEC 62040-3 sets out how to specify and test the performance of a UPS. Its classification code packs three answers into one short string: what the output depends on, what the waveform looks like, and how the voltage behaves when something changes.<\/p>\n\n\n\n

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VFI \u2013 SS \u2013 111<\/strong>: input dependency, output waveform, dynamic performance.<\/p>\n<\/blockquote>\n\n\n\n

Part 1: input dependency<\/h2>\n\n\n\n
Code<\/th>Meaning<\/th>Typical topology<\/th><\/tr><\/thead>
VFD<\/td>Output voltage and frequency both depend on the input<\/td>Offline \/ standby<\/td><\/tr>
VI<\/td>Output voltage is regulated, but frequency follows the input<\/td>Line-interactive<\/td><\/tr>
VFI<\/td>Output voltage and frequency are independent of the input<\/td>Online double conversion<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Only VFI isolates the load from voltage and frequency changes on the grid. If you are not sure why that matters, see online vs offline systems<\/a>.<\/p>\n\n\n\n

Part 2: output waveform<\/h2>\n\n\n\n

Two letters: the first for normal mode, the second for battery mode.<\/p>\n\n\n\n

Letter<\/th>Meaning<\/th><\/tr><\/thead>
S<\/td>Sinusoidal, within limits with both linear and reference non-linear loads<\/td><\/tr>
X<\/td>Meets the sinusoidal limits with linear loads, but not non-linear ones<\/td><\/tr>
Y<\/td>Non-sinusoidal output<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

SS<\/strong> means the output stays sinusoidal in both modes, so you do not lose waveform quality when the system goes onto battery.<\/p>\n\n\n\n

Part 3: dynamic performance<\/h2>\n\n\n\n

Three digits, each a class from 1 to 3, in this order:<\/p>\n\n\n\n

    \n\n
  1. Mode change:<\/strong> for example from normal mode to battery mode.<\/li>\n\n\n
  2. Linear load step:<\/strong> a sudden change in a linear load.<\/li>\n\n\n
  3. Non-linear load step:<\/strong> a sudden change in a non-linear load.<\/li>\n\n<\/ol>\n\n\n\n
    Class<\/th>How the output behaves<\/th>Suits<\/th><\/tr><\/thead>
    1<\/td>Stays within the tightest tolerance curve, with no meaningful break<\/td>Sensitive and critical loads<\/td><\/tr>
    2<\/td>Allows a short disturbance, in the order of 10 ms<\/td>Most IT supplies with some hold-up<\/td><\/tr>
    3<\/td>Allows a longer disturbance<\/td>Less sensitive loads<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    So VFI-SS-111<\/strong> describes an online unit with a sinusoidal output in both modes and the tightest class on all three tests. The exact curves are in the standard; ask for the test report if a project depends on them.<\/p>\n\n\n\n

    What the code does not tell you<\/h2>\n\n\n\n