Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 66 replies
  • Subscribers 32 subscribers
  • Views 27856 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Minimum values of insulation resistance

Wombat
What is the science behind the 1 MOhm minimum insulation resistance? What is the basis for this particular value?
Parents
  • 0

    gkenyon:




    Wombat:

    What is the science behind the 1 MOhm minimum insulation resistance? What is the basis for this particular value?




    It matches perfectly the permitted touch current for double insulated appliances (0.5 mA) for a supply voltage U0 of 500 V.


    For U0 = 1000 V, 1 MΩ would lead to a touch current of 1 mA (the accepted threshold of perception).


    These values align with Clause 5.2.7 in BS EN 61140 Protection against electric shock - common aspects for installation and equipment, which states the limit of steady-state touch current to be 0.5 mA AC or 2 mA DC (higher levels are permitted in some circumstances).


    In fact, there is some science behind these values, and they are based on the IEC 60479-series of standards Effect of current on human beings and livestock which is used as a reference standard for developing electrical safety standards - BS EN 61140 is itself based on this series of standards.


     




    Have just been going through BS EN 61140. Clause 5.2.7 has limitation of steady state touch current 
    a) For touch current, the following values are proposed:
    – a steady-state current flowing between simultaneously accessible conductive parts not exceeding the threshold of perception, 0,5 mA a.c. or 2 mA d.c. under normal operating conditions;
    – values not exceeding the threshold of pain 3,5 mA a.c. or 10 mA d.c. may be specified under abnormal or fault conditions.


    So if we are using the 1000VDC insulation resistance test voltage and minimum insulation resistance of 1MOhm we get a maximum allowable current of 1mA which is half the threshold of perception.

    In the real situation we would have a cable with 1MOhm resistance operating at 230VAC and giving a leakage current of 0.23mA, which is again half the threshold of perception.


    Looking at IEC/TR 60479*1 clause 5.2 the threshold of reaction is 0.5mA for ac and clause 6.1, 2mA for dc. This equates to the physiological effect "Perception possible but usually no started reaction" for ac and "Slight pricking sensation possible when making, breaking or rapidly altering current flow" for dc.


    An insulation resistance minimum of 500kOhms would still be within the Standard requirements. 


    Why then does BS7671 require 1MOhms for minimum insulation resistance?

     

Reply
  • 0

    gkenyon:




    Wombat:

    What is the science behind the 1 MOhm minimum insulation resistance? What is the basis for this particular value?




    It matches perfectly the permitted touch current for double insulated appliances (0.5 mA) for a supply voltage U0 of 500 V.


    For U0 = 1000 V, 1 MΩ would lead to a touch current of 1 mA (the accepted threshold of perception).


    These values align with Clause 5.2.7 in BS EN 61140 Protection against electric shock - common aspects for installation and equipment, which states the limit of steady-state touch current to be 0.5 mA AC or 2 mA DC (higher levels are permitted in some circumstances).


    In fact, there is some science behind these values, and they are based on the IEC 60479-series of standards Effect of current on human beings and livestock which is used as a reference standard for developing electrical safety standards - BS EN 61140 is itself based on this series of standards.


     




    Have just been going through BS EN 61140. Clause 5.2.7 has limitation of steady state touch current 
    a) For touch current, the following values are proposed:
    – a steady-state current flowing between simultaneously accessible conductive parts not exceeding the threshold of perception, 0,5 mA a.c. or 2 mA d.c. under normal operating conditions;
    – values not exceeding the threshold of pain 3,5 mA a.c. or 10 mA d.c. may be specified under abnormal or fault conditions.


    So if we are using the 1000VDC insulation resistance test voltage and minimum insulation resistance of 1MOhm we get a maximum allowable current of 1mA which is half the threshold of perception.

    In the real situation we would have a cable with 1MOhm resistance operating at 230VAC and giving a leakage current of 0.23mA, which is again half the threshold of perception.


    Looking at IEC/TR 60479*1 clause 5.2 the threshold of reaction is 0.5mA for ac and clause 6.1, 2mA for dc. This equates to the physiological effect "Perception possible but usually no started reaction" for ac and "Slight pricking sensation possible when making, breaking or rapidly altering current flow" for dc.


    An insulation resistance minimum of 500kOhms would still be within the Standard requirements. 


    Why then does BS7671 require 1MOhms for minimum insulation resistance?

     

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines