Author Topic: Transformers used in class II 'double insulated' mains powered devices  (Read 331 times)

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Offline e100

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Are the transformers used in things phone chargers (where there is no earth pin) any different from regular transformers?

I've looked at various teardown videos and I haven't seen any design features that to my untrained eye look 'extra safe'.
« Last Edit: August 22, 2018, 04:01:12 pm by e100 »
 

Offline capt bullshot

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Re: Transformers used in class 2 'double insulated' mains powered devices
« Reply #1 on: August 22, 2018, 03:48:17 pm »
The difference is in the insulation materials, and air / creepage distances. Most mains connected transformers are made this way, no matter if the appliance is earthed or not. So most of them won't look any special, that's right.

So e.g. the primary and secondary pins must have enough distance between them, for SMPS transformers often a special "triple insulated" wire is used, other (line frequency) transformers have bobbins with different chambers for primary and secondary or layers of insulation material between the primary and secondaries.
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Offline e100

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Re: Transformers used in class 2 'double insulated' mains powered devices
« Reply #2 on: August 22, 2018, 05:00:41 pm »
The difference is in the insulation materials, and air / creepage distances. Most mains connected transformers are made this way, no matter if the appliance is earthed or not. So most of them won't look any special, that's right.

So e.g. the primary and secondary pins must have enough distance between them, for SMPS transformers often a special "triple insulated" wire is used, other (line frequency) transformers have bobbins with different chambers for primary and secondary or layers of insulation material between the primary and secondaries.

Searching for class II/double insulated information is a confusing mess of multiple standards hidden behind $200 paywalls, however I found this transformer datasheet
https://www.rapidonline.com/pdf/525391.pdf
The description says "As a safety isolating transformer as specified in EN 61558-2-6 for the safe electrical isolation of the input and output sides. The transformer is suitable for creating SELV and PELV circuits because of the limit on the output voltage."

Wikipedia (https://en.wikipedia.org/wiki/Extra-low_voltage#Separated_or_safety_extra-low_voltage_(SELV) says "
A SELV circuit must have:
 - Electrical protective-separation (i.e. double insulation, reinforced insulation or protective screening) from all circuits other than SELV and PELV (i.e. all circuits that might carry higher voltages)
- Simple separation from other SELV systems, from PELV systems and from earth (ground)"

So does that mean that a transformer built to EN 61558-2-6 is "double insulated"?
 

Offline capt bullshot

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Re: Transformers used in class II 'double insulated' mains powered devices
« Reply #3 on: August 22, 2018, 05:57:21 pm »
I don't have access to the quoted 61558-2-6 standard, only 61558-1, anyway these standards are quite unreadable for the average EE. Dealing with the standards must be your day job to understand them, or go and ask a lawyer (or VDE person) of your least mistrust.

In general saying something is build according to a particular standard doesn't say too much, as there are often multiple classes or levels of whatever this standard shall achieve described within these standards. So for your question, it isn't safe to assume a transformer is "double insulated" when it is stated it is build to 61558-6-2 alone. If it says "safety class II" according to standard ..., it might be double insulated to achieve this, but there might be also other ways to achieve this class. Often this is quite difficult to find out by reading the standard, as these often written in kind of legalese.
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Offline e100

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Re: Transformers used in class II 'double insulated' mains powered devices
« Reply #4 on: August 22, 2018, 08:29:35 pm »
I don't have access to the quoted 61558-2-6 standard, only 61558-1, anyway these standards are quite unreadable for the average EE. Dealing with the standards must be your day job to understand them, or go and ask a lawyer (or VDE person) of your least mistrust.

In general saying something is build according to a particular standard doesn't say too much, as there are often multiple classes or levels of whatever this standard shall achieve described within these standards. So for your question, it isn't safe to assume a transformer is "double insulated" when it is stated it is build to 61558-6-2 alone. If it says "safety class II" according to standard ..., it might be double insulated to achieve this, but there might be also other ways to achieve this class. Often this is quite difficult to find out by reading the standard, as these often written in kind of legalese.

Element14 lists a small number of 'class II' transformers, for example
http://au.element14.com/block/sim100/transformer-safety-isolating/dp/1131568
which has the double insulated square within a square symbol on the side.
Confusingly the datasheet shows the same picture but doesn't mention 'class II' but says "Safety transformer to IEC 61558-2-6, DIN EN 61558-2-6,
VDE 0570 part 2-6"
http://www.farnell.com/datasheets/12654.pdf

 


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