In daily experience it is difficult to come across exposed conductive parts as, for electrical safety reasons, they are separated from the user and electrically isolated in protective cases. [1]

However, in the design and use of electrical devices we must manage conductive parts with different electrical potential, with even very high voltage differences.

These conductors are separated by insulating material, which prevents the passage of electric charge from the conductor of higher potential to the one with lower potential.

When designing the system made up of insulating material and conductors, the risk of formation of electric arcs in the air [2] and creeping currents on the surface of the insulation material must be considered.

To limit these phenomena, during the design phase we intervene on the distances between exposed conductive parts.

In technical regulations, the separation distances between conductive parts at different electrical potential are called electrical spacing. [3]

A distinction is made between shortest distance in air between two conductive parts which is called clearance [4] and the shortest distance along the surface of an electrically insulating material between two conductive parts called creepage distance [5].

Industrial electrical devices (non-Ex), where there is no product standard, can refer to the IEC 60664-1 standard, in which specific tables establish the relationship between conductor voltages and the minimum required values of clearance and creepage distances.In increased safety devices compliant with IEC/EN 60079-7, a similar table is found differentiated for protection levels (EPL) Gb and Gc. [6] 

In table 1, we see an example limited to the case of the EPL Gb.

Table1 - Example with values extracted from table 2 of IEC 60079-7:2015 for deviceswith EPL Gb and for some voltage values [7]

In the case of creepage distances, as reported in table 1, it is necessary to know that the insulating material varies in its effectiveness in countering the formation of creeping currents. Its ability to oppose the tracking phenomenon is measured through the CTI (comparative tracking index) [8] and the materials are divided into groups I, II, IIIa and IIIb. For this reason, the group of materials I reported in table 1, corresponds to a smaller creepage distance, in other words the quality of the material allows the designer of the device to reduce the distance between the conductors.

In Figure 1, we see some useful examples to understand the difference between the clearance and the creepage distance. In particular, for geometric reasons and for the definition given in the regulations, the creepage distance is never less than the clearance between the same two conductors, at most these two distances can coincide as in the first example shown in figure 1.

Figure 1: Some examples of clearance and creepage distances as reported by the IEC/EN 60079-7 standards.

As can be seen from Table 1, for high voltage values the standard requires that the creepage distance is greater than the relative clearance.

By taking advantage of the geometric definition, it is possible to increase the creepage distance with the use of ribs or grooves in the insulating material. For example, the use of collars around cylindrical conductors forces the creeping current to follow a longer path consisting of a descent and a subsequent ascent (Figure 2). This path is greater than the distance that would be obtained if the surface of the insulating material between the conductors was flat. The distance X shown in figure 2, represents the minimum value below which the electric arc would tend to skip the depression, making the collar irrelevant in this case. Therefore, a greater depression is required for devices with EPL eb due to their higher level of protection.

Similarly, a rib increases the creepage distance by forcing the creeping current to follow an upward and downward path. This second method has the advantage of also increasing the distance in the air, as seen in the third example of Figure 1.

Figure 2: example of collars around cylindrical conductors to create grooves.

Although at first glance it may seem that the topic of electrical spacing only concerns the designer of electrical devices, the wider topic of electrical connections also involves the user and those who work in the systems.

When installing the equipment in the field, care must be taken that the connections cannot loosen and, for this reason, the screw connections are equipped with anti-loosening devices such as spring washers, growers or similar. In certain cases, to increase the clearance and the creeping distances, when the voltages are high, the assembly of accessory bulkheads is required, as can happen in some cases in modular terminals or other Atex certified components. It is therefore important to read with due attention both the use and maintenance manual and the limitations reported in the equipment or component certificate.

Notes and bibliographical references

[1] An example could be the metal part inside a terminal, which for obvious functional reasons, must have the screw within reach of a tool, or the entry for the conductors in a plug to be wired. Even on printed circuits where the conductive tracks normally run under an insulated layer (solder resist) there are points where live parts are exposed, for example, in correspondence with the discrete components that are soldered onto a printed circuit.

[2] The air, even if dry, has a weak conductivity, due to electrically non-neutral particles; this conductivity increases significantly with the presence of humidity as water is a better conductor than air

[3] CEI EN 60079-0:2013-09 par 3.51

[4] CEI EN 60079-0:2013-09 par 3.51.1

[5] CEI EN 60079-0:2013-09 par 3.51.2

[6] In comparison to the 4th edition of IEC 60079-7, EPL Gc has been incorporated into the table by adding what originally was the Ex-nA protection method (non-sparking devices) of the IEC/EN 60079-15. It should be noted that, unlike non-Ex industrial standards, the validity of the table here is broader and not linked to a specific type of device.

[7] By voltage we mean the Ur.m.s. in a.c. or d.c.

[8] Please refer to the article The tracking phenomena and the Comparative Tracking Index – CTI published on the Cortem website