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Old 15th June 2014, 01:26 AM   #1
beanbag is offline beanbag  United States
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Default Class 2 safety question

Hello folks,

I have built a Class D amp with Hypex modules and SMPS. Since I was using unbalanced RCA inputs, the instructions said to have the 3rd prong of the power cord not attached, and to attach the outers of the RCA inputs to the case. They also said to electrically connect one of the stand-offs from the power supply to the chassis. ("J7 is connected to secondary ground with a 100nF capacitor. Connect J7 to chassis with a metal
spacer for optimum EMI performance. ")

The instructions further say to have class 2 double insulation. I am not sure how to implement this. My case is bare aluminum. The heatsink of the amplifier modules are connected to the case, although the transistors on them appear to be insulated. The SMPS is already sort of connected to the chassis via the standoff. Other parts of the SMPS are at least 1/4" away from bare metal.

I guess one concern is if somehow a high voltage touches the case. In that case, won't it fry my DAC, which is connected to the RCA?

If it's for shock protection of me touching the case, then should the case be coated with an insulator? But wouldn't my RCA outer and SMPS standoff still be connected to it?
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Old 15th June 2014, 10:37 AM   #2
Julf is online now Julf  Europe
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It is purely for electrical safety. Do you want/need to conform to formal safety regulations? If so, you pretty much have to connect the 3rd prong of the power connector to the chassis (and the instructions don't actually tell you you shouldn't, they just indicate precautions if you do). In that case, you should probably try to float the input connectors with the resistor and capacitor as described in the instructions.
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Old 15th June 2014, 10:54 AM   #3
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Class 2 wiring describes the wiring between the power source and the connected equipment, usually loudspeaker wires. If you have mains gong to your amplifier box and it is metallic, exposed parts must be earthed for your own safety and to comply with regulations.
There's more;
The NEC defines a Class 2 circuit as that portion of the wiring system between the load side of a Class 2 power source and the connected equipment. Due to its power limitations, a Class 2 circuit is considered safe from a fire initiation standpoint and provides acceptable protection from electrical shock.
The Code defines a Class 3 circuit as that portion of the wiring system between the load side of a Class 3 power source and the connected equipment. Since the Class 3 circuits has higher levels of current than for Class 2 circuits, it specifies additional safeguards to provide protection from an electric shock you might encounter on the job site.
Power for Class 2 and Class 3 circuits is limited either inherently (in which no over current protection is required) or by a combination of a power source and overcurrent protection.
The maximum circuit voltage is 150VAC or VDC for a Class 2 inherently limited power source and 100VAC or VDC for a Class 3 inherently limited power source. The maximum circuit voltage is 30VAC and 60VDC for a Class 2 power source limited by over current protection, and 150VAC or VDC for a Class 3 power source limited by over current protection.
For example, heating system thermostats are commonly Class 2 systems and the majority of small bell, buzzer, and annunciator systems are Class 2 circuits. Class 2 also includes small intercom phone systems in which a battery and the ringing circuit supply the voice circuit.
Where a Class 2 circuit is routed a distance where voltage drop becomes a problem (by not providing a voltage that will energize the equipment), Class 3 circuits are sometimes used to provide the necessary voltage and current. Class 3 circuitry and accessories can be designed to alleviate the problem of excessive voltage drop.
See WHAT IS CLASS 2 WIRING
Use your head and all will be fine.
Class 2 and 3 circuits are defined as the portion of the wiring system between the power source and the connected equipment. Because of the power limitations of Class 2 circuits, many consider them to be safe from a fire initiation standpoint and to provide an acceptable level of protection from electrical shock. - See more at: Class 2 and 3 Circuits | EC Mag
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Support for Flying Mole Class D, PMC DS-001 (Hypex),Valve Equipment and designs both new and old. www.jonsnell.co.uk
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Old 15th June 2014, 11:31 AM   #4
dmills is offline dmills  United Kingdom
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That is UL class 2, but what is clearly being discussed here is class II (Double insulated appliances), not the same thing at all.

Personally I take the view that all DIY needs to be class I if in a metal case, there are just too many ways to get class II lethally wrong.

Regards, Dan.
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Old 15th June 2014, 11:57 AM   #5
Julf is online now Julf  Europe
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Quote:
Originally Posted by dmills View Post
Personally I take the view that all DIY needs to be class I if in a metal case, there are just too many ways to get class II lethally wrong.
I have to agree.
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Old 15th June 2014, 12:28 PM   #6
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Quote:
Originally Posted by dmills View Post
Personally I take the view that all DIY needs to be class I if in a metal case, there are just too many ways to get class II lethally wrong.
That's good advice.
Even if the european protection class II in would allow metal casings without PE (protective earth), the catalogue of details how to meet the requirements of double isolation is really no fun.
No matter if the Hypex SMPS meets the requirements, most DIYers will mess the safety by their surrounding implementation when trying to realize class II. Even most EE are struggling hard to get it right.

Already a class I set up has enough points to mess, but usually remains by far more forgiving than a wrongly done class II build.
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Old 15th June 2014, 01:08 PM   #7
dmills is offline dmills  United Kingdom
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Metal cased class II is seen (Lots of commercial hifi stuff), but it is very subtle to design, I cannot reccommend going there.

Stick to class I, it is harder to screw up and can be at least as quiet if done right.

Regards, Dan.
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Old 15th June 2014, 01:15 PM   #8
Julf is online now Julf  Europe
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Quote:
Originally Posted by dmills View Post
Metal cased class II is seen (Lots of commercial hifi stuff), but it is very subtle to design, I cannot reccommend going there.
Just to translate for non-brits - read the "very subtle to design" as "bloody hard"
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Old 15th June 2014, 01:16 PM   #9
beanbag is offline beanbag  United States
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umm, sorry, what do I need to do for Class I?

Here is the Hypex wiring guide:
http://www.hypex.nl/docs/wiring.pdf
"When using RCA inputs do NOT connect chassis to safety earth to avoid ground loops. Amplifier must be build according to double insulation safety standards"

Also This:
http://www.hypex.nl/docs/appnotes/earth_appnote.pdf

Last edited by beanbag; 15th June 2014 at 01:29 PM.
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Old 15th June 2014, 01:29 PM   #10
Julf is online now Julf  Europe
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Quote:
Originally Posted by beanbag View Post
umm, sorry, what do I need to do for Class I?
Connect the chassis to mains earth.
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