Creepage

Only very small amounts of oil, less than one drop with most small and medium-sized ball and roUer bearings, are sufficient to provide a full EHL film (5). In such cases, a small amount of grease or oil mist balances lubricant loss by vaporization, creepage, and throw-off. With high surface speeds and heavy loads, however, much larger lubricant feed is needed for cooling and makeup. 

The minimum clearance and creepage distances must be maintained for conductors as specified. 

Anti-loosening, anti-rotating type terminal arrangement with specified clearance and creepage distances... 

Locations falling within this category can employ motors that are more economical than the other types discussed above. lEC 60079-14 has defined the basic requirements for such enclosures which are obviously less stringent than the others. In addition to maintaining specified creepages and clearances between the rotating and the stationary parts, the following are the two main requirements specified for such enclosures ... 

Special provisions are laid down in lEC 60079-0 and lEC 60079-1 for motors required for such locations in view of fluctuating degrees of humidity and temperature. Such locations are defined with a surface temperature limit of 150°C where coal dust can form a layer, or 450°C where it is not expected to form a layer. Otherwise, other details are generally the same as for flameproof motors type Ex d , according to lEC 60079-1. For variations in length of paths, gaps, widths, creepage and clearance distances, the reader should consult these Standards. 

To contain the temperature of the electrical circuits within safe limits for a particular temperature class of the surroundings, the maximum current rating for a minimum size of a conductor is also stipulated in ICC 60079-1 I. The constructional requirements also stipulate the minimum clearances and creepage distances in air between the conducting parts of all the intrinsically safe L lectrical eirctiits. 

Special-purpose motors such as increased safety motors, flame-proof or explosion-proof motoi s must be checked for gaps, clearances and creepage distances of all the mating parts forming flame paths. The construction of these motors must follow lEC 60079 as noted in the list of standards. 

The clearance and creepage distances between the terminals and between each terminal and the ground. 

To comply with the rated insulation level, all the current-carrying components forming part of the assembly should have clearances and creepage distances according to their relevant standards whereas busbars and busbar connections must have the distances noted below. 

The clearances and creepage distances should be maintained as shown in Tables 28.4 and 28.5. These values can be reduced when ... 

This check may be carried out both at the works, after final assembly, and at the site after the installation before conducting routine tests or energizing the assembly. The ehecks can be carried out at a lower voltage to detect any shorting links or spanners left inadvertently on live terminals, weak insulation or insufficient clearance or creepage distances between the phases or phase to ground conduetors. In the ease of a defect the same must be rectified immediately. 

The clearances may be verified as in Table 28.4, whereas for creepages Table 28.5 may be followed. 

Table 15.1 Recommended values of minimum creepage distances for a VT or a CT...

Pollution level Minimum creepage distance between phase and ground mm per kV(r.m.s.) (phase to phase)... 

The rated frequency, insulation systems and the requirement of creepage distances will generally remain the same as for a voltage transformer (Section 15.4,1). For the remaining parameters, the following may be noted. 

These values are considered for an altitude of up to 2000 m for LT and 1000 m for HT systems. For higher altitudes to achieve the same level of dielectric strength, the values of clearances and creepage distances, as given in Tables 28.4 and 28.5, may be increased by at least 1 % for every 100 m rise in altitude. 

Table 28.S Creepage distances for enclosed indoor air insulated busbars as in BS 159 ...

Rated voltage e Minimum creepage distance to ground Minimum creepage distance between phases... 

The iibove figures are only indicative, and may be considered as a minimum tor a bus system that is dry and free from dust or any contamination, which may influence and reduce the elfective creepage over time. These ereepages may be increased for damp dirty or contaminated locations. 

The creepage distance between the opposing phases of the input (between the -I- and - dc and between the HI and H2 (or hot and neutral) must be 3.2mm minimum. 

The conductivity of a surface varies with the long-term pollution exposure in the operating atmosphere and its average humidity. The previously mentioned creepage spacings change for different applications, such as industrial, telecom, etc. The designer must refer to the appropriate specification. 

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