Newsgroups: sci.engr.electrical.compliance
Subject: Re: Couple EN 60950 questions
References: <[email protected]>
Harold Hallikainen  writes:

>     We just got a report back from the safety lab and I thought I'd get
> additional opinions on a couple items. The product is a small theatrical
> light dimmer that plugs into a standard wall outlet. It has four outlets
> for driving lamps.
> 
> #1 - Overcurrent protection. The dimmer has four outlets and four 10 amp
> circuit breakers. The 10A breakers (in the hot side of the line, not the
> neutral) are between the incoming AC and the dimming circuitry for each
> channel. They thus protect the internal wiring past the breaker, the
> triac, the choke, and the outlet for each channel. 

Sound good up to this point to me. I have designed some
dimmer circuit, bu never made them to a commercial product and
through the safety lab inspections. In one of my own circuits
I have made protection so that I had per channel fuses
(for serious short circuit protections) and then one main breaker
for protecting against input overload.

> The power inlet is
> either a Neutrik PowerCon (20A) or an IEC60320 C14 inlet (10A). The lab
> is suggesting that we are not complying with 2.7.1, yet it appears to me
> that as pluggable equipment type A, we may rely upon the protection of
> the wall socket to protect the equipment (insuring that 40A is not drawn
> through the power inlet). Comments?

For case of IEC 10A inlet my views is also that you can't rely on
protection of the wall socket fusing. That inlet is rated for
10A current, but the protection on the wall sockets in Europe
are generally of higher value. Uusally wall sockets are
fused with 10A or 16A fuses in continental Europe and with 13A
in UK. This means that depending on the country you would be drawing
13A or 16A current through 10A reted connector without any protections
doing anything. I would se this as a safety hazard. So if the equipment
uses that 10A inlet, my view is that there should be a 10A main 
breaker to make sure that you are not overloading the power input.
This would be the good and safe engineering practice. On case
that all the outputs are 10A rated and maximun load is 10A,
I would think that you just need that 10A main breaker
(and not any 10A per channel protections, that's the point
of putting two similar 10A breakers in series... Would save some money).

For that 20A Neutrik PowerCon, I am not sure if that would comply
without any extra protection, But also in this case I would
say that a 16A main overload protection would be a good and
sound practice. And then using those 10A per channel breakers.

The general industry practice in this kind of portable dimmers
seems to be that there is almost always a main breaker for input
current overload protection and then besides that per channel
protection (breakers or fuses). This applies to well designed
portable theatrical dimmer systems I have used and seen.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/