Hi Vincent,

1)
Class III definition is only applicable to complete product and no to output or 
other circuitry. You can have a class III product what means that it is 
supplied by SELV voltages and that there are internally no voltages above SELV 
limits generated. Laptop adapter cannot be classified as class III product, 
since it has primary voltage inside. Output voltage cannot be classified class 
III since this is not intention of class III definition. Laptop supplied by 
external adapter can be classified as class III in case no internal hazardous 
voltages are generated. Bear in  mind that 19VDC/DC converter can generate 
internal voltages above SELV limits.

Your statement about output current and let-go current is completely wrong. 
SELV voltages do not need to comply with touch current (only to PE in case of 
floating circuit). For example, on USB output you can get 1A between plus and 
minus but this is still safe to touch since it is SELV voltage.


2)
Isolation transformer differs from output transformer since secondary windings 
are completely isolated from primary windings. Most often we are taking about 
reinforced insulation. Isolation transformer is usually also 1:1 transformer 
and it just helps to reduce touch current (leakage current). Step down 
transformer means that it generates on secondary side low voltage, that can be 
safe to touch (SELV).


3)
Yes, SELV voltage needs to be isolated from primary voltage and therefore you 
need isolation transformer. Every switch mode power supply has internally 
isolation transformer.


I hope this helps.

Best regards,
Bostjan

From: Vincent Lee [mailto:000008e6c8d35910-dmarc-requ...@ieee.org]
Sent: Thursday, March 8, 2018 4:17 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Isolation Transformer & Class III electrical protection


Hi all,

Good day,

(1) If Class III electrical protection is defined as appliance with voltage 
output less than SELV (30Vrms based on IEC 60950 IT equipment), so can a laptop 
AC - DC power adaptor with output rating 19Vrms, 2.37A be considered as Class 
III, given that output current, 2.37A is much more than 10mA Threshold "Let-Go" 
current ?


(2) How does Isolation Transformer different from the usual Step-Down Voltage 
Transformer?


(3) And how does Isolation Transformer helps to achieve Class III electrical 
protection as stated in page 3 of 
https://www.excelsys.com/wp-content/uploads/2011/09/ApplicationNoteAN1102-ClassIvsClassII.pdf

Hope to hear from you soon. Thank you & have a nice day ahead.

Vincent



Regards, Vincent
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