Table of contents

• EN61558 (Safety of transformers, reactors, power supply units and combinations thereof) and other safety standards
• What is Clearance?
• What is Creepage?
• Graphical representation of Creepage and Clearance
• Indirect paths
• Some common examples in transformers
• How to determine the correct Creepage and Clearance?
  • Voltage across and between windings
  • Insulation types
  • Overvoltage categories
  • Pollution degree
  • CTI of insulating materials
  • Altitude
• A typical transformer

One of the aspects often neglected during the initial phases of electronic design, is the definition of the Creepage and Clearance safety distances required by standards.

In many cases, this leads to the necessity of a late reiteration of the design which involves the transformer, the PCB, EMC tests etc., resulting in delay and extra cost.

When this aspect isn’t addressed at the beginning of the design process, it’s usually due to an underestimation of its potential impact, or simply to a lack of knowledge of the topic.

This article serves as a general, non-comprehensive orientation on the Creepage and Clearance safety distance requirements for transformers.

The majority of the concepts discussed here can be applied to power supplies (PSUs – Power Supply Units) as well, meaning that more categories of readers can benefit from this guide.
But it is important to clarify that all the examples and terminology in this guide are referred to transformers.

The transformer is usually one of the few components, among others like optocouplers, Y capacitors and PCBs, which are responsible for guaranteeing an adequate insulation.

Before tackling safety distances in transformers, here’s a brief overview of the regulatory framework.

EN61558 (Safety of transformers, reactors, power supply units and combinations thereof) and other safety standards

The standard EN 61558 is applied in absence of explicit requirements in each product’s specific standard.
It defines all the safety requirements of transformers, including Creepage and Clearance (Cr & Cl).

In recent years many technical committees are aligning their standards to EN 60664 “Insulation coordination for equipment within low-voltage supply systems”, so far as is reasonable according to the reference products and sectors.
“Horizontal” standards like EN 60664 have, in fact, the function of guidelines for technical committees, in order to standardize the requirements for homogeneous topics as much as possible.
For this reason, it’s common for the requirements of different standards to be essentially aligned with each other.

Moreover, EN 60335, EN 62368, EN 60950, EN 61347, EN 60598, EN 62115 and many other standards include explicit references to EN 61558 for the compliance of transformers.

Though, in some cases, the specific product standards (e.g. electromedical, intrinsic safety – Ex i, etc.) have electrical safety requirements significantly different than those of EN 61558.

That said, let’s move on with the main topic of this article: Creepage and Clearance.

What is Clearance?

Clearance (Cl) is the shortest distance in air between two conductive parts.

The evaluation of clearance is done measuring the length of the shortest path through air, considering that this path doesn’t pass through insulating bodies, like insulating tape, sleeving, coating, plastic parts etc.

Also note that two parts of insulating material close together, even if glued, don’t have the same effect as a one-piece part.
In fact, Creepage and Clearance paths can pass through the junction between the two parts.

The conductive parts (or conductive bodies) of the simplest transformer are the primary winding, the secondary winding and the metallic or ferrite core.

For a transformer like this, in most cases we talk about safety distances that have to be guaranteed between the primary winding and the secondary winding.

The possible conductive parts of other windings or components electrically connected to the primary winding, must guarantee the same minimum distances towards the secondary winding that are required between primary and secondary winding (and vice versa).

What is Creepage?

Creepage (Cr) is the shortest distance along the surface of an insulating material between two conductive parts.

The same considerations made for Clearance also apply to Creepage, except that the Creepage path doesn’t pass through air, but it’s bound to the surface of insulating bodies.

Graphical representation of Creepage and Clearance

The figure below can help to better understand Creepage and Clearance through a simple example, where conductive parts are represented by coppery cubes, while insulating parts are those coloured green (to symbolize a PCB).

Some common examples in transformers

Creepage and Clearance are, by definition, the shortest distances identified between all the possible paths between the condutive parts of interest.

The following drawings show some examples of paths that are often considered when evaluating Creepage and Clearance between primary winding and secondary winding of transformers.

But first, a clarification about the type of wire used for the windings:

the enamel of a simple enamelled wire is not relevant towards the safety distances, so this type of wire must be considered as uninsulated, hence conductive;

on the contrary, wires with solid and continuous insulation, like those with solid multi-layer insulation (TIW), those with high thickness secure enameling (FIW) or electrical cables with appropriate thermal index, have to be considered insulated with regard to the evaluation of Creepage and Clearance.
As a result, the only parts of a winding made with an insulated wire that are relevant for the evaluation of Creepage and Clearance, are those from which the insulation is removed: usually the ends soldered on pins and the pins themselves.

For simplicity’s sake, the following examples will only mention the TIW wire to exemplify any kind of wire with solid insulation.

Example 1

The drawing shows a transformer with primary winding (orange wire) and secondary winding (yellow wire) separated by yellow insulating tape, represented in cross-section.

In this case a simple enamelled wire has been used for both windings, meaning that insulating tape is the only insulation separating the turns of the two windings with regard to Creepage and Clearance.

Therefore, the shortest paths between the turns of the two windings passing around the insulating tape, like Cr1 and Cl1, are to be considered.

Examples 2 and 3

This drawing shows an end of the primary winding insulated by a tube and soldered to a pin.

As in example 1, the wire used for the secondary winding is not TIW.
As for the primary winding, we have two cases:

1. besides the tube, the wire is simply enamelled;
2. besides the tube, a TIW wire has been used.

In the first case, the shortest distances (Cr2 and Cl2) could be those between the end of the tube on the primary winding and the closest point on the turns of the secondary winding.

In the second case, the end of the primary winding is fully insulated up to the soldering on the pin, so the shortest distances (Cr3* e Cl3*) could be those between the pin on the primary side and the closest point on the turns of the secondary winding.

Cl3* is represented by two different arrows, indicating that Creepage and Clearance paths that have to be verified can be many.

Example 4

This is and example of indirect paths.

In some cases the shortest distances can be those between the terminals of the primary winding and the terminals of the secondary winding, passing through the core.

In this example

• distances Cr4-A and Cl4-A are those between a terminal of the primary winding and the closest point on the core, respectively through air and along the bobbin surface;
• distances Cr4-A and Cl4-A are those between a terminal of the secondary winding and the closest point on the core, respectively through air and along the bobbin surface;
• the path lengths are calculated as follows: Cr4 = Cr4-A + Cr4-B and Cl4 = Cl4-A + Cl4- B.

Example 5

This other example of indirect paths applies in case of windings made with simple enamelled wire.

In this case the shortest paths could be between the primary winding turns and the secondary winding ones, passing through the core.

The path lengths are calculated as follows: Cr5 = Cr5-A + Cr5-B and Cl5 = Cl5-A + Cl5-B.

Example 6

Other possible paths to consider are those between the primary winding turns and the secondary winding pins.

The drawing shows two paths of this kind, between a secondary winding pin and the closest point on the primary winding (which happens to be under the insulating tape) made with simple enamelled wire.

In this example the secondary winding is made with TIW wire, whereas if it had been only enamelled, the shortest path would have been between the primary turns and the secondary turns, passing around the insulating tape as in example 1.

Clearance and Creepage paths overlap because there aren’t distances through air shorter than those along the bobbin surface.

Example 7

Positioning other components next to the transformer may affect the compliance of Creepage and Clearance.
When evaluating Creepage and Clearance, it is necessary to also consider the conductive parts of possible components electrically connected to the windings of the transformer.

In this example

• the primary winding is made with simple enamelled wire;
• the secondary winding is made with TIW wire;
• a capacitor is electrically connected to the secondary winding.

A possible path to consider for evaluating Clearance is between the primary winding turns (under the tape) towards the uninsulated top of the capacitor, passing through the core.

How to determine the correct Creepage and Clearance?

Now that we know what Creepage and Clearance are, it remains to understand how to determine adequate safety distances for our application.

This guide provides no specific indications for defining the distances needed for your application.
To do this, it is necessary to refer to the requirements of the relevant standards.

Nonetheless, here’s a description of the elements to consider, and how these affect the required Creepage and Clearance.

These factors are:

• Voltage across windings, beetween adjacent windings and between primary and secondary winding groups;
• Required insulation type (functional, basic, supplementary, double/reinforced);
• Overvoltage category (I, II, III, IV);
• Pollution degree (P1, P2, P3);
• CTI of the insulating materials;
• Maximum altitude, where over 2000m;
• Operating frequency;
• Homogeneous/inhomogeneous electric field.

The next paragraphs cover each of these factors, except the last two, which have a limited or negligible impact for the most common switching frequencies.

Voltage across and between windings

First of all, the required Cr & Cl depend on the voltages across the windings and across the insulations between windings.

The higher these voltages, the higher the required Cr & Cl.

In particular, voltages having an impact on Creepage and  Clearance are:

• Maximum RMS voltage and repetitive peak voltage measured across the windings or series of windings;
• Maximum RMS voltage and repetitive peak voltage measured across the input/output insulation, that is between any primary side pin and any secondary side pin, whether at the main rated maximum voltage (e.g. for 90-264V wide range it usually is 230V or 240V, not 264V), at no-load and at maximum load. During tests, the output pole on which the highest voltage is measured shall be earthed.
• Maximum RMS voltage and repetitive peak voltage measured between each couple of physically adjacent (separated only by insulating material) and not interconnected windings, at the main rated maximum voltage, at no-load or at maximum load.

Repetitive peak voltage has an impact on Creepage and Clearance if exceeding 750Vpk and if double insulation (see the paragraph “Insulation types” is obtained through insulated wires.
In this case, partial discharge type tests are also required.
Random transient voltages must be disregarded.

Insulation types

There are five types of insulation with four different safety levels.
The higher the insulation level, the higher the required Creepage and Clearance.

The insulation between the windings connected on the mains side and the output ones, or between any winding and another one, can be of the following types:

• Functional: doesn’t require any minimum Creepage, Clearance or insulation thickness.
  The common winding wire enamel is an example of functional insulation.
  This insulation type can be applied between windings intended to be mutually connected or in components having no safety function.
• Basic: basic standard requirement; there is no minimum thickness required for the insulation.
• Supplementary: further insulation with minimum required thickness, for maintaining a certain degree of safety in case of failure of the basic insulation.
• Double: basic + supplementary.
• Reinforced: a single insulation system with higher minimum required thickness, guaranteeing the same safety level of double insulation.

Safety transformers, which are by far the most commonly applied in power electronic equipment, shall comply with EN 61558-1 and EN 61558-2-6, and also EN 61558-2-16 if they are switching transformers.
Double or reinforced insulation is required for all safety transformers.

As a typical example, double or reinforced insulation is required for mains-powered transformers with an Extra Low Voltage (ELV – Extra Low Voltage) output that can come into contact with people.

Double or reinforced insulation is also required when no direct contact is possible but certain insulations, Creepage, Clearance etc. are not guaranteed against the output conductors.

• OVC-I: overvoltage category of the equipment for connection to circuits in which measures are taken to limit transient overvoltages to an appropriately low level (powered by isolating transformer).
  Examples of such equipment are those containing electronic circuits protected to this level.
  However, unless the circuits are designed to take the temporary over-voltages into account, equipment of overvoltage category I cannot be directly connected to the mains supply.
• OVC-II: overvoltage category of the equipment to be supplied from the fixed installation, but not permanently connected to it.
  Typically, mobile equipment connected to the mains via plug.
  Examples of that equipment are transformers for household appliances, telecommunications, toys, and similar loads.
• OVC-III: overvoltage category of the equipment used in fixed installations and for cases where the reliability and the availability of the equipment is subject to special requirements.
  Examples of such equipment are transformers in fixed installations and transformers for industrial use with permanent connection to the fixed installation.
  Typically, fixed equipment connected downstream of a low voltage mains (<500Vac) utility energy meter.
• OVC-IV: overvoltage category of the equipment used at the origin of installation.
  Examples are transformers in fixed installations of power plants or immediate to such installations.

The standard EN 61558 requires OVC-III for general use transformers.

Pollution degree

Is the transformer installed in a dry and clean place, or there may be conductive dust and moisture?
These elements affect the required Creepage.

The more the environment is polluted, the higher the required Creepage, while there are no effects on Clearance.

Standards define three pollution degrees:

• P1: no pollution or only dry, non-conductive pollution occurs.
  P1 is applied to encapsulated transformers and similar.
  Creepage requirements for this pollution degree are obviously not too restrictive, but time-consuming type tests are necessary in order to be able to apply these distances.
• P2: only non-conductive pollution occurs, except that occasionally a temporary conductivity caused by condensation is to be expected.
  P2 degree, which is the most common condition, is assigned to transformers having a reasonably hermetic enclosure, but not completely sealed.
• P3: conductive pollution occurs, or dry non-conductive pollution occurs which becomes conductive due to the condensation which is to be expected.

Referring to the whole transformer, though, is not quite correct.
It would be more appropriate to mention the parts of the transformer for which Creepage and Clearance requirements are actually relevant.

As an example, for a resin encapsulated (and properly tested) transformer, a P1 Creepage can be applied even a P3 environment.
This is because the windings and the core won’t come into contact with the pollution in the environment, since they’re incorporated into resin.

CTI of insulating materials

Some insulating materials are prone to surface flashover at higher voltages than others.
The Comparative Tracking Index (CTI) defines the property of withstanding voltage without triggering surface discharges.
CTI is correlated to this voltage.

According to CTI, materials are classified by groups:

A higher CTI allows shorter Creepage, while Clearance is not affected.

Altitude

A typical transformer

In this case, Creepage and Clearance of little less than 6mm are usually sufficient, according to most standards.

This situation covers the vast majority of cases in power electronics.

Nonetheless, it is important to understand that it’s always necessary to make specific assessments, since there may be particular characteristics that have to be considered, in addition to minor standard requirements not addressed in this article.