Calculating Creepage and Clearance Early Avoids
Design Problems Later
of the most common errors uncovered by product safety engineers stems
from manufacturers and designers failing to fully investigate a product's
creepage and clearance distances.
It is not unusual for manufacturers to find that
a product fails the creepage and clearance distance test because of
miscalculations or simply because the distance between two components
was overlooked. Design engineers, especially printed circuit board (PCB)
designers, are often not aware of the reasons for using creepage and
clearance distances. Selecting the appropriate tables in the standard
and applying them properly to a design are key to avoiding problems
Last-minute failure can also arise because design
engineers do not seek input from the product safety engineers in the
early design stages. Designers sometimes assume that all safety issues
relating to creepage and clearance have been addressed, only to discover
spacing problems once the product is built.
Creepage Distance. Creepage is the shortest path
between two conductive parts (or between a conductive part and the bounding
surface of the equipment) measured along the surface of the insulation.
A proper and adequate creepage distance protects against tracking, a
process that produces a partially conducting path of localized deterioration
on the surface of an insulating material as a result of the electric
discharges on or close to an insulation surface. The degree of tracking
required depends on two major factors: the comparative tracking index
(CTI) of the material and the degree of pollution in the environment.
Used for electrical insulating materials, the CTI provides a numerical
value of the voltage that will cause failure by tracking during standard
testing. IEC 112 provides a fuller explanation of tracking and CTI.1
Tracking that damages the insulating material normally occurs because
of one or more of the following reasons:
Humidity in the
Presence of contamination.
Altitude at which
equipment is to be operated.
Clearance Distance. Clearance is the shortest
distance between two conductive parts (or between a conductive part
and the bounding surface of the equipment) measured through air. Clearance
distance helps prevent dielectric breakdown between electrodes caused
by the ionization of air. The dielectric breakdown level is further
influenced by relative humidity, temperature, and degree of pollution
in the environment.
When designing a switch-mode power supply for
use in information technology (IT) equipment, a typical rule of thumb
is to allow an 8-mm creepage distance between primary and secondary
circuits, and a 4-mm distance between primary and ground. If these dimensions
are allowed for during the design stage, there is a high probability
(95%) that no failure will occur with respect to creepage or clearance
when the final product is submitted for test.
Working Voltages. A working voltage is
the highest voltage to which the insulation under consideration is (or
can be) subjected when the equipment is operating at its rated voltage
under normal use conditions. The appropriate creepage and clearance
values can be determined from the figures provided in the relevant tables
in EN 60950.2 These values must sometimes
be calculated. To use Tables IIV (2H, 2J, 2K, and 2L of the standard),
the following factors must be considered: determination of working voltages,
pollution degree of the environment, and the overvoltage category of
the equipment's power source.
When measuring working voltages, it is important
to measure both peak and root-mean-square (rms) voltages. The peak value
is used to determine the clearance, and the rms value is used to calculate
creepage. For example, if one measures a peak voltage of 670 V between
two pins of a switching transformer in a switch-mode power supply, the
clearance distance between primary and secondary circuits must be calculated
using Table I. If the unit is powered via 240 V mains and has a pollution
degree of 2, the figures in the center row (marked 300 V rms sinusoidal)
and center column (since the mains voltage is >150 V and < 300
V) are used to establish the required clearance distance. In this case,
the value for reinforced insulation is 4 mm. One then turns to Table
II (Table 2J of EN 60950), which provides additional clearance based
on the working voltages and pollution degree. (The middle column was
used for calculating this example.) The appropriate row in that column
covers the actual repetitive peak insulation working voltage. In this
example, the value would be 0.8 mm for reinforced insulation. Adding
the two figures together gives a total of 4.8 mm clearance distance.
Similarly, if a voltage of 337 V rms was measured between the two pins
of the switching transformer, Table IV (2L of the standard) must be
used to calculate the creepage distance between the primary and secondary
circuits. Assuming pollution degree 2 and material group IIIb, the required
creepage distance for basic insulation would be 3.5 mm using linear
interpolation. For reinforced insulation, the values for creepage distances
are double the values provided in the table for basic insulation. In
this case, the required creepage for reinforced insulation would be
Basic, and Supplementary Insulation
Rms or Dc
II, IIIa, or IIIb
from the appropriate
is permitted between the nearest two points, the calculated spacing
being rounded to the next higher 0.1-mm increment.
Table IV. Table 2L of the standard provides
minimum creepage distances (creepage distances in millimeters).
The use of these tables is explained in sections
126.96.36.199.4 of EN 60950. Measurements should be accurate and
repeatable and should also consider the end application.
Pollution Degrees and Overvoltages
Pollution degree is divided into four categories.
The following definitions are based on those in IEC 60664.3
1. No pollution or only dry, nonconductive pollution occurs. The
pollution has no influence (example: sealed or potted products).
2. Normally only nonconductive pollution occurs. Occasionally
a temporary conductivity caused by condensation must be expected (example:
product used in typical office environment).
3. Conductive pollution occurs, or dry, nonconductive pollution
occurs that becomes conductive due to expected condensation (example:
products used in heavy industrial environments that are typically
exposed to pollution such as dust).
4. Pollution generates persistent conductivity caused, for instance,
by conductive dust or by rain or snow.
The overvoltage, also known as installation, category
is also divided into four categories according to IEC 60664.
I. Signal level (special equipment or parts of equipment), with
smaller transient overvoltages than overvoltage category II.
II. Local level (appliances and portable equipment), with smaller
transient overvoltages than overvoltage category III.
III. Distribution level (fixed installations) with smaller transient
overvoltages than overvoltage category IV.
IV. Primary supply level (overhead lines, cable systems, etc.).
This category is not relevant to most product standards.
Typically, most standards are based on conditions
being pollution degree 2 and overvoltage category II. It is important
to note that as working voltage, pollution degree, overvoltage category,
and altitude increase, the creepage and clearance distances also increase.
The altitude is particularly important when testing to EN 61010.4
Creepage and Clearance in Practice
Each part of a circuit must be studied to determine
the necessary insulation grade. Table 2G in EN 60950 describes common
applications of insulation. For example, establishing the required creepage
and clearance between a primary circuit and an ungrounded safety extra
low voltage (SELV) circuit requires reinforced insulation. By measuring
and establishing both the working voltage and the pollution degree,
the appropriate row and column in Table 2H (and if necessary Table 2J)
determine the minimum clearance distance needed. For one test, the internal
components and parts in both primary and secondary circuits are subjected
to a steady force of 10 N, and certain minimum clearance distances must
be maintained during the test.
Because the primary circuit is an internal circuit
connected directly to the external supply mains, this circuit typically
contains hazardous voltage. A secondary circuit, which has no direct
connection to primary power, may or may not be hazardous. Nonhazardous
circuits are classified as SELV.
Dc input products, however, can be treated in
one of two ways. They can be considered as being fed by an extra-low-voltage
circuit, or as hazardous secondary voltages. This would mean that the
clearances could be calculated using Table
III rather than Table I, requiring slightly smaller clearance distances.
Dc input products can also be considered as being fed by SELV secondary
circuits, depending upon the end application. If isolation is needed,
then Table III of the standard is used. However, if isolation is not
required in the end application, then clearances are waived, and only
operational insulation is required.
As IT products continue to get smaller, it is
more important than ever to have a good and calculated PCB design that
not only reduces electromagnetic interference emissions, but that also
reduces creepage and clearance problems. Where shortage of space on
a PCB is an issue, especially between primary and SELV circuits, techniques
such as slots or grooves can be used to attain desired creepage distance.
Slots must be wider than 1 mm; otherwise, they are not considered acceptable.
For a groove (>1 mm wide) the only depth requirement is that the
existing creepage plus the width of the groove and twice the depth of
the groove must equal or exceed the required creepage distance. The
slot or groove should not weaken the substrate to a point that it fails
to meet mechanical test requirements.
Another solution is to design the PCB so that
components are mounted flat on the board rather than positioned vertically.
This layout overcomes problems that might arise from the 10- N push
test required in EN 60950. A minimum of 8 mm separation between primary
and secondary circuits also prevents problems. When semiconductors operating
at hazardous voltages are mounted on grounded or floating heat sinks,
certain precautions must be taken to ensure compliance with EN 60950.
If heat sinks happen to be live (and they can be), they should be marked
accordingly to warn service personnel. Generally, a semiconductor's
plastic enclosure is considered as operational (necessary for correct
operation of the equipment) or, in some cases, as only basic insulation.
Therefore, depending on the heat sink's grounding arrangement, the semiconductor
requires either basic or reinforced insulation.
It is equally important to consider creepage
and clearance even when using UL-recognized power-switching semiconductors.
Although these products carry a recognition mark, the manufacturer's
data sheets must be examined to ensure that the components are suitable
for the intended
The working voltages of the circuit must be taken
into account. Transistors with built-in reinforced insulation (body
thicker than 0.4 mm) must also still meet the spacing requirements at
their legs. Some designers mistakenly assume that UL certification eliminates
the need for further examination.
In some casesin particular for switch-mode
power suppliesthe design topology can lead to the need for higher
creepage distance in the switching transformer. In such situations,
the use of a wider margin tape (also known as saddle tape) may not be
practical; therefore, the use of multilayer insulated wire (also known
as triple-insulated wire) is highly recommended. When using triple-insulated
wire, it is important to remember that such wire must satisfy the requirements
described in Annex U of EN 60950. Lack of adequate creepage and clearance
distance between a component in a primary circuit to a component in
the SELV circuit is a common cause of product failure. A typical short-term
solution is to place an insulating material, such as Mylar sheet, with
appropriate thickness and dielectric withstand voltage, between the
two parts, ensuring that the sheet is mechanically secure. Room-temperature
vulcanizing sealant, a silicone paste cured at room temperature, or
a similar material, is used not only as a means of bonding components
together for mechanical purposes, but also to overcome clearance problems.
However, materials that are used to compensate for clearance problems
must be UL recognized, particularly if a product is to be sold in North
Calculation and measurement of creepage and clearance
distances are among the most important parts of all safety standards,
and therefore it is important for design engineers to consult the product
safety engineers throughout the design stages to avoid any failure at
the test house before a product is launched into the market.
Creepage and clearance distances not only apply
to the PCB, but also to the components (especially magnetic components)
that are mounted on the PCB. It is also important to note that as working
voltage, pollution degree, overvoltage category, and altitude increase,
both the creepage and clearance distances also increase.
1. IEC 112:1979, "Method for Determining
the Comparative and the Proof Tracking Indices of Solid Insulating Material
under Moist Conditions," International Electrotechnical Commission,
2. BS EN 60950:2000, "Safety of Information
Technology Equipment," British Standards Institute (BSI), United Kingdom.
3. IEC 60664:1980 "Insulation Co-ordination
within Low-Voltage Systems Including Clearances and Creepage Distances
for Equipment," International Electrotechnical Commission, Brussels.
4. BS EN 61010-1:1990, "Safety Requirements
for Electrical Equipment for Measurement,
Control, and Laboratory Use, Part 1: General Requirements," BSI, United
Homi Ahmadi is approvals manager for Cortech
Systems (Simi Valley, CA). He can be reached at Hahmadi@cortechsys.com.