Hi Stuart,
I have come across this issue before, and if the PSU is a switch mode
device without an earth connection i.e. double insulated this does not
necssarily constitute a safety issue or is the result of a failure (In my
opinion though it is a quality issue at the very least as it frightens the
hell out of users). Below is an extract from an e-mail on the subject that
came from a safety lab which explains the situation (all references are to
EN60950-1).
=======================================
=======================================
====================================================
1. SELV circuit(see cl 1.2.8.6): A secondary circuit which is so designed
and protected that under normal operating conditions and single fault
conditions, its voltages do not exceed a safe value(42.4Vpeak or 60Vdc).
2. Limited current circuit(see cl 1.2.8.7): A circuit which is so designed
and protected that, under both normal operating conditions and single fault
conditions, the current which can be drawn is not hazardous.
And in clause 2.1 we can find that the operator is permitted to have access
to following bare parts with protected against electric shock:
- bare parts of SELV circuits; and
- bare parts of Limit Current Circuits; and
- TNV circuits under the conditions specified in 2.1.1.1. (not a concern in
this case, so this won't be considered)
As we know, when a Y capacitor connected between primary circuit and
secondary circuit, the voltage between the point of secondary circuit
connected with Y capacitor (normally the '-' terminal of output) and ground
is always very high even more than 100V, but it does not mean it is
hazardous since the current which can be drawn will be limited by the
approved Y capacitor, and according to clause 2.4 of this standard we must
perform the Limited Current Circuit test, when the test result comply with
the requirements of the standard, the product is still safe.
If no Y capacitor connected between the primary circuit and secondary
circuit (just like a linear adaptor), the primary circuit and secondary
circuit are completely separated by isolating transformer, the output
voltage is floated to primary circuit and ground, it is not necessary to
perform the Limited Current Circuit test.
Of course the voltage between '+' and '-' terminal of the adaptor must
comply with SELV requirements, and touch current test should be performed
no matter there are Y capacitors between primary and secondary circuit or
not.
=======================================
==========================================================================
Section 2.4.2 of EN60950-1 specifies that for frequencies not exceeding
1kHz, the steady state current drawn through a non-inductive resistor of
200 ohms +/- 10% shall not exceed 0.7mA peak or 2mA dc.
Hope this helps.
Kind regards,
Chris.
Stuart Miller -
Ablewisp
<[email protected]> To
Sent by: Emc-Pstc <[email protected]>
[email protected] cc
Subject
18/04/2006 12:01 Portable hard disk power supply
Hi group
I hope I can get some advice on an issue with a power supply. It was
supplied, as a separate unit, with a Maxtor portable hard disk drive
that we use for backups in the office (UK based). The drive sits on a pc
case.
The supply is manufactured by DVE, model number 0421S. It provides 12V
dc at 3A. Rated input is 100-240V, 50/60Hz, 1.2A.
Its CE marked, UL listed and has a TuV mark stating BSEN60950. It's
marked as double insulated.
I guess its around 18 months old and has had an "easy" life in the
office, with no visible case damage.
After connecting the data cable (Firewire, already connected to the pc)
to the drive, I touched its metal case while touching the outer of the
dc connector and got an unpleasant tingle.
I measured 230 V from the metal case to the outer of the dc connector
(not connected to the portable hard disk).
To isolate the drive from ac ground, I removed the data cable from the
drive and measured 230V from the metal case to ac power ground (the
drive is insulated from the pc case by plastic feet). Current was around
0.2mA.
When the dc connector is connected, the drive performs normally.
The drive was removed for secure storage when we left the office over
the Easter holiday. In normal use it remains connected to one pc for
weeks at a time. It would be exciting to move the drive from pc to pc!
I couldn't find a recall notice for the DVE part number on Google.
I'm interested in possible failure modes (I'm not a switch mode power
supply expert) that could result in this shock hazard. I assume its due
to degrading components and will get worse!
Many thanks in advance
Stuart Miller
Ablewisp Limited
www.ablewisp.com
ph: 01803 843502
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