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Sunday, 24 June 2012

Testing surge protected extension leads

Most modern PAT testers have a facility for quickly testing surge protected extension leads. You just plug both ends of the power cord into the tester (using a short IEC lead) and quickly carry out the Earth Continuity, Insulation Resistance and Polarity (wiring) tests.



Earth Continuity test
 
When testing Earth Continuity, the tester passes a current down the Earth wire and measures the resistance. If it is a long extension lead, then the user has to adjust the PASS limit accordingly - this will be the subject of another blog.

Wiring test
 
The Wiring test consists of checking wheter the LIVE and NEUTRAL wires are correctly wired and do not have any open or short circuits.

Insulation Resistance test

The Insulation Resistance test is normally carried out at 500V as recommended in the IEE Code of Practice. While this is fine for normal extension leads, there can be complications when testing surge protected extension leads. To understand this, we need to look a bit closer at how this test is done.

The PAT tester shorts the LIVE and NEUTRAL wire together and applies 500V to this with respect to the Earth wire. Then the current flowing is measured and used to work out the Insulation Resistance. The lower the current, the higher the Insulation Resistance and the safer the appliance is. In most cases the current measured will be ZERO, resulting in a reading that will be the maximum that the PAT tester can measure. Often whis will be displayed as >20M, indicating that the Insulation Resistance is greater than 20 million ohms.

Surge protected extension leads

On surge protected appliances, there are protection devices known as transient supressors that are fitted between the LIVE and EARTH wires and the NEUTRAL and EARTH wires. At normal mains voltages of 230V these devices are virtually "invisible" and dont interfere with normal operation. However if the voltage were to go higher, then they will start to operate and clamp the voltage to a safe value to protect any appliances plugged into the extension lead.

This "clamping voltage" is usually less than 500V. When an Insulation Resistance test is carried out, the 500V used for the test will cause these devices to operate and take current. This current is interpreted as an Insulation Resistance failure although the extesnion lead is behaving exactly as it has been designed to do.

What does the user do when faced with this? 

Do you FAIL the extension lead because the tester is saying FAIL, or do you PASS it because you suspect that the FAIL is due to the surge-protection devices used inside the extension lead?

The IEE Code of Practice recommends the following course of action if the user gest a FAIL when testing a surge protected extension lead at 500V.

1. Retest at 250V if the PAT tester has this option. At this voltage the protection device does not operate.

2. If the PAT tester does not have this option then PASS the extension lead as the FAIL is due to the surge-protection devices used inside the extension lead.

Do all surge protected extension leads FAIL when tested at 500V?

Not necessarily. The 500V test voltage can vary from 450V upto 600V on most PAT testers. The surge protection device used can be from 400 to 500 V. So it is possible for PAT testers testing at "500V" to PASS surge protected extension leads.

Summary

Surge protected extension leads can present a "false FAIL" when tested for Insulation Resistance at 500V. The IEE Code of Practice recommends retesting at 250V if possible.






1 comment:

  1. Question, you said at the start "you just plug both ends of the power cord into the tester (using a short IEC lead) and quickly carry out the Earth Continuity, Insulation Resistance and Polarity (wiring) tests".

    Why do you use the word "quickly"? Is there a reason why we can't do it at a normal pace?

    ReplyDelete