G8MNY > TECH 15.03.24 08:15z 101 Lines 4519 Bytes #158 (0) @ WW
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Subj: Portable Appliance Testing
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Sent: 240315/0808Z @:GB7CIP.#32.GBR.EURO #:3298 [Caterham Surrey GBR] $:3298_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jun 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
In the UK all commercially used portable mains equipment has to be tested to
ensure it meets basic electrical safety tests.
The 2 basic tests on a 13 Amp plug & test probe are:-
1/ VOLTAGE
Resistance of Live & Neutral to Earth > 500kê @ 500V (DC voltage > 2x RMS).
This test ensures the mains insulation is not too damp or about to break down.
A basic test arrangement might look something like this..
ÚÄÄÄÄÄÄÄÄÄ¿ ____
ÚÄÄÄÄ68kÄÄÄ´Pass/FailÃÄÄo oÄÄÄÄÂÄÄ< L Portable
Mains 500V ³ /³\ ³ Test ³ Appliance
PSU DC ³ DC ³ mA ³ ÀÄÄ< N under test &
³ ÀÄÄÄÄÄÄÄÄÄÙ switched on
EÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ< E
³
ÀÄÄÄÄÄÄÄÄ> Probe for exposed metal
2/ EARTH CURRENT
Earth case Bond current test up to 25 Amps ( >1.5x fuse rating) with a low
voltage (e.g. 6V) this equates to a 0.1ê lead R. This test ensures the earth
wiring can withstand the full fault condition fuse rupturing current. (e.g. not
earthing with a few strands of wire, that would pass simple CONTINUITY TEST)
A basic test arrangement might look something like this..
ÚÄÄÄÄÄÄÄÄÄ¿ ____
ÚÄÄÄ0.15êÄÄÄ´Fail/PassÃÄÄo oÄÄÄÄ<E Portable
Mains 6V ³ /³\ ³ Test Appliance
PSU AC ³ AC ³Amps³ under test
³ ÀÄÄÄÄÄÄÄÄÄÙ
EÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ> Probe for exposed metal
More complex testers will measure the values & have different ratings for more
delicate & lower fuse rated items. See DANGERS below.
Some testers do a high voltage flash test (e.g. 3kV) to ensure the mains
circuit insulation is adequate (e.g. unearthed TV). Also a standard "SMALL
FINGER" type probe my be used to simulate what could be accessed by a child.
ÚÄÄÄÄÄ¿
(From Harry M1BYT) These test would normally be used on equipment ³ ÚÄ¿ ³
described as "DOUBLE INSULATED", marked by a square box within a ³ ÀÄÙ ³
square box symbol. ÀÄÄÄÄÄÙ
The flash test is done between any exposed metal on the appliance & the mains
lead. For instance - testing a double insulated electric drill, you would place
the flash test probe on the drills chuck, with the drill plugged into the
instrument. That would confirm that the chuck is adequately insulated from the
mains & that it could not become live.
The term "double insulted" refers to there being two effective insulating
barriers between any mains wiring or live parts & exposed metal work.
Insulation between the windings of the motor & the shaft plus a second
barrier in the form of nylon gearing + case before the steel chuck.
DANGERS
As you can appreciate these tests can be dangerous for some types of equipment.
For instance many electronic items (PC, Audio & Video Mixers, etc.) do not have
25A current carrying capacity PCB earth tracks to all of their exposed metal
parts, then PCB earth tracks & fine wires & connectors can BE DAMAGED or even
VAPORISED!
FALSE READINGS
Items that run hot like irons & kettles must have there insulation tested when
cold & when hot, for the test to have any meaning.
Some items can fail a test due to false testing concepts. e.g. A long 13A mains
lead wound on a steel winding drum, can fail the earth bond test (as above)
when the wire is coiled, but pass when unwound, due to the increase in
inductance of the cable on the steel drum.
Smoothing Capacitor faults like framing on an electrolytic that caused high
earth capacitive leakage currents (>100mA) that can be difficult to locate...
ÚÄ´>ÃÄÂÄÄÄÂÄÄÄÄ +ve
LÄÄ´ ³ _³_ An insulation fault causes the metal
³ Ú´>ÃÙ |+³ | case to earth. It is normally at -ve,
NÄÄ)Ä´ |===| but only via an electrolytic effect
³ À´<ÿ ~³~\ inside the case.
³ ³ ³ ³ A DC megger test to earth showed all was OK !
ÀÄ´<ÃÄÁÄÄÄÁÄÄ)ÄÄ -ve
³
EÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄ E
See my bul on "Earths", "Constant Voltage Transformers", "Edgcume Peebles Earth
Loop Z", "Maplin Mains Meter 2000MU-UK", & also "Mains Power Protection".
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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