# How to determine the Air Clearance for Floated Secondary Circuit

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#### sankui

According to clause 8.9.1.12, a secondary circuit derived from a SUPPLY MAINS will normally be overvoltage category I according to IEC 60664-1 if the MAINS PART is overvoltage category II.

The 8.9.1.12 also says, where a secondary circuit is not earthed and is derived from a SUPPLY MAINS, the circuit shall be subjected to the requirements for primary circuits in Table 13 and Table 14.

However, I don't know if the statement of overvoltage category I also applies when checking the values from Table 13 and 14.

For example:

- Rated MAINS = 240V, 50-60Hz;

- MAINS overvoltage category II;

- Pollution degree 2;

- The floating secondary circuit powerd from insulation transformer = 120V;

Is the required clearance between such secondary circuit and unearthed enclosure (two MOOP) 4.0 mm @ CAT II or 2.0 mm @ CAT I?

Thank you very much!

Last edited by a moderator:

#### Stijloor

Super Moderator
Re: How to determine the air clearance for floated secondary circuit?

A Quick Bump!

Can someone help?

Thank you very much!!

#### Peter Selvey

Super Moderator
Re: How to determine the air clearance for floated secondary circuit?

In principle use the values for Cat II.

The theory is that mains voltage is not a clean sine wave, but has transients on top, e.g. just turning off the vacuum cleaner or fluorescent light will produce inductive spikes on the mains circuit, and safety insulation needs to handle these spikes as well as the mains.

If the secondary is earthed, such spikes will be significantly reduced when measured in the secondary circuit. However, if the circuit is floating (really floating) the spike can be transferred through pri-sec capacitive coupling without any significant reduction.

In reality the secondary might be earthed via intentional capacitors which are low impedance with respect to high speed transients. But the standard is not clear if this is an acceptable means for changing from Cat II to Cat I.

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#### sankui

Re: How to determine the air clearance for floated secondary circuit?