Technology

Issue No. 10

14 May, 2007

Dear Readers!
We have already seen what direct and indirect contacts mean in one of the initial issues. Let us brush up the basic definitions of the same and later understand the measures of protection against both types of contacts.

PROTECTION AGAINST DIRECT AND INDIRECT CONTACT
Direct Contact:
A direct contact refers to a person coming in contact with a conductor which is live in normal circumstances.

Indirect contact:
An indirect contact refers to a person coming into contact with an exposed conductive part which normally is not alive, but has become alive accidentally (due to insulation failure or some other cause).

Protection of people according to IEC 364:
a) The time / current curves given by the IEC 479 define the IEC 364 time / touch voltage boundary curves. b) The curves depend on the type of premises (dry, damp or wet) c) the maximum automatic tripping time is 5 seconds

Conventional maximum safety voltage UL:

UL

%Prem

Atmosphere

Examples

50V

81 %

dry

Entertainment facilities, shops, offices, printing houses….

18 % 25V

damp

Washing areas, job sites, breeding areas…

Did U Know ?
1%
IEC 61140 standard has renamed “Protection against direct contact” & “Protection against indirect contact” with the terms “Basic protection” & “Fault protection” respectively.

very damp

Swimming pool, bathroom, shower…

Measures of protection against direct contact:
Two complementary measures are commonly used as protection against the dangers of direct contact: a) The physical prevention of contact with live parts by barriers, insulation, inaccessibility, etc. b) Additional protection in the event that a direct contact occurs, despite or due to failure of the above measures. This protection is based on residualcurrent operating device with a high sensitivity (IÄn ? 30 mA) and a low operating time. These devices are highly effective in the majority of case of direct contact. All the physical protective measures are preventive, but...