ABC Inventors - electrical safety device - Page 4

put finger to keyboard and composed:



I'm not familiar with the relevant standard, but can this device
possibly meet the requirements for isolation between housing and
neutral? Furthermore, if the active and neutral conductors are
reversed in the GPO or in an extension cord, wouldn't the device
itself render the appliance unsafe? Granted, Protex may interrupt
power to the appliance in this case, but current (6mA?) would need to
pass through the user's body in order for it to do so. Elsewhere in
the patent application I see a design variation incorporating an
optocoupler. Perhaps that is a better approach.


- Franc Zabkar

The circuit arrangement looks to me to perform a similar task to that
described for the voltage dependant ELCB as described by Pat Ward. The
main difference seems to be that the vELCB used a ground referenced
metal frame and Protex uses a neutral referenced metal part(17).

QUOTE:
ELCB = Earth Leakage Circuit Breaker. There are two types of ELCB, the
voltage operated device and the differential current operated device.
For the convenience of this article only (and at the risk of causing
even more confusion) I will refer to these as vELCB and iELCB. vELCBs
were first introduced about sixty years ago and iELCBs were first
introduced about forty years ago.

The principle of operation of the vELCB is as follows. Under normal
conditions the closed contacts of the vELCB feed the supply current to
the load. The load is protected by a metal frame, such as in an
electric cooker. The vELCB also has a relay coil, one end of which is
connected to the metal frame and one end connected directly to ground.
A shock risk will arise if a breakdown in the insulation occurs in the
load which causes the metal frame to rise to a voltage above earth. A
resultant current will flow from the metalwork through the relay coil
to earth and when the frame voltage reaches a dangerous level, e.g. 50
volts, the current flowing through the relay coil will be sufficient
to activate the relay thereby causing opening of the supply contacts
and removal of the shock risk.

As can be seen from the above description, this type of ELCB is
essentially a voltage sensing device intended to detect dangerous
touch voltages. The level of shock protection provided by the vELCB
was somewhat limited as these devices would not provide shock
protection in the event of direct contact with a live part. An
additional problem with the vELCB was its tendency to be tripped by
earth currents originating in other installations.

Pat Ward is managing director of Western Automation R & D based in
Ballinasloe. The company is a specialist designer and manufacturer of
RCD products. Pat has been an active member of technical committees in
ETCI, IEC and CENELEC since 1991 and has been involved in the drafting
of several international standards for RCD & RCMs. He has also
participated in working groups within IEC addressing problems of
reliability and EMC requirements for RCDs.


The full article is here
http://www.westernautomation.com/pages/demystify.htm

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Hi Ross,
Any reason why you have set your posts to be not archived?
They are useful and it would be nice to have them archived on Google.

Regards
Dave :)

"David L. Jones"





 ** The evil that men do lives on after them .....

    ( full apologies to the bard )



 BTW

 RH has his "no archive" buried in the header details  -  very cute.




..........    Phil

Hi David,

I can't imagine that my posts will be of any value years down the
track. Anyway, a great deal of any particular person's postings are
just a waste of space on any archive.

Where are you located? Email me direct for any other details - don't
forget the munged address.

On Tue, 17 May 2005 21:21:22 GMT, Ross Herbert



Maybe, but there can be a lot of useful stuff, too. I occasionally
follow up old postings in the Google archives, and I often receive
enquiries from people all over the world looking for parts,
datasheets, or other information that I have accumulated over the
years. In fact I've had three enquiries in the past fortnight.


- Franc Zabkar

Franc Zabkar wrote:


The groups archive is an incredibly useful resource.
Standard search practice for me is to search Google and then the Google
Groups archive, but often I go straight to Google Groups search knowing
that I'll get much better results. It's only a single mouse click away
and very often brings up links and discussions that you can't find with
the regular Google web search.

Dave :)