Plugpacks and grounding

Contemplating how to power custom-built gear from the mains safely,
and within the regs. So if I buy a 'bench power supply', I can connect
the LVDC output to more or less anything, including a bare breadboard
that I'm working on. But AFAIUI, that doesn't apply to a plugpack,
which requires anything it powers to be 'approved double insulated'
(see (*) following). So what's the characteristic of the bench supply
that allows it to be used in this way?



(*) I was curious about how plugpack outputs go in terms of
input-output leakage, so I checked the voltage between the output and
local ground, for a couple of relatively new 9VDC SMPS units. Both of
them showed open circuit voltage of > 90VAC, and a short circuit
current to ground of around 100 uA. Not close to dangerous current
level, but enough for a significant tingle, and enough to discourage
use as a general supply around the workbench.

 

[Chris Jones]

On 06/12/2014 18:34, Bruce Varley wrote:

Contemplating how to power custom-built gear from the mains safely,
and within the regs. So if I buy a 'bench power supply', I can connect
the LVDC output to more or less anything, including a bare breadboard
that I'm working on. But AFAIUI, that doesn't apply to a plugpack,
which requires anything it powers to be 'approved double insulated'

No, that is not the case. Otherwise they would have to use different
connectors on the output of the plugpacks.

[snip]

 

On Sun, 07 Dec 2014 00:05:28 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

On 06/12/2014 18:34, Bruce Varley wrote:
Contemplating how to power custom-built gear from the mains safely,
and within the regs. So if I buy a 'bench power supply', I can connect
the LVDC output to more or less anything, including a bare breadboard
that I'm working on. But AFAIUI, that doesn't apply to a plugpack,
which requires anything it powers to be 'approved double insulated'

No, that is not the case. Otherwise they would have to use different
connectors on the output of the plugpacks.

[snip]

OK, that's useful to know. In that case, is there any problem with
grounding one side of a SMPS plugpack output? I can't see why there
would be but a more informed answer would be useful.

 

[Phil Allison]

nor...@googlegroups.com wrote:

Contemplating how to power custom-built gear from the mains safely,
and within the regs. So if I buy a 'bench power supply', I can connect
the LVDC output to more or less anything, including a bare breadboard
that I'm working on. But AFAIUI, that doesn't apply to a plugpack,
which requires anything it powers to be 'approved double insulated'

** Completely wrong and damn silly.

What an AC plug-pak powers is not an "appliance" at all and has no electrical safety rules to meet.

It can be made of plastic or metal, have batteries inside or not, be connected to other devices that may or may be earthed to the supply safety ground.

So what's the characteristic of the bench supply
that allows it to be used in this way?

** Bench PSUs are excluded from the normal rules for appliances - since they are considered to be electrical test equipment. Such equipment is expected to be used ONLY by qualified persons and may output dangerous voltages.


..... Phil

(*) I was curious about how plugpack outputs go in terms of
input-output leakage, so I checked the voltage between the output and
local ground, for a couple of relatively new 9VDC SMPS units. Both of
them showed open circuit voltage of > 90VAC, and a short circuit
current to ground of around 100 uA. Not close to dangerous current
level, but enough for a significant tingle, and enough to discourage
use as a general supply around the workbench.

 

[Phil Allison]

nor...@googlegroups.com wrote:

(*) I was curious about how plugpack outputs go in terms of
input-output leakage, so I checked the voltage between the output and
local ground, for a couple of relatively new 9VDC SMPS units. Both of
them showed open circuit voltage of > 90VAC, and a short circuit
current to ground of around 100 uA. Not close to dangerous current
level, but enough for a significant tingle, and enough to discourage
use as a general supply around the workbench.

** Most SMPS plug-paks will have a EMI suppression cap that connects from the AC side ( usualy the filter cap) to the output common. This cap is specially rated for the purpose ( IIRC it's class Y2) and has a value not exceeding 2nF.

So the output plug floats at half the AC voltage, measured on a high enough impedance meter compared to safety ground.

Most iron transformer plug-paks do the same thing, but the capcitance involved is merely strays from primary to secondary and has a value of around 100pF.

FYI: External PSUs normally have to be fully type approved AND comply with MEPs before going on sale.

However, if the device being powered is not intended for domestic use AND it needs more than one DC rail supplied from the PSU - it need not comply with either requirement.



..... Phil

 

[BuckyBalls]

On 6/12/2014 2:33 PM, Bruce Varley wrote:

On Sun, 07 Dec 2014 00:05:28 +1100, Chris Jones
lugnut808@spam.yahoo.com> wrote:

On 06/12/2014 18:34, Bruce Varley wrote:
Contemplating how to power custom-built gear from the mains safely,
and within the regs. So if I buy a 'bench power supply', I can connect
the LVDC output to more or less anything, including a bare breadboard
that I'm working on. But AFAIUI, that doesn't apply to a plugpack,
which requires anything it powers to be 'approved double insulated'

No, that is not the case. Otherwise they would have to use different
connectors on the output of the plugpacks.

[snip]

OK, that's useful to know. In that case, is there any problem with
grounding one side of a SMPS plugpack output? I can't see why there
would be but a more informed answer would be useful.

You can get earthed plugpacks.
http://www.jaycar.com.au/productView.asp?ID=MP3022
http://www.altronics.com.au/powertran/power-packs/?type_1=ac-to-ac-plugpacks&prdf=5&srt=7&brws=2