The important thing to remember is the function of the fuse. 99%+ of the time
it is for fire protection, not essential overcurrent protection. Most of us
rely on the customer wall outlet to be properly protected per NEC or 950.
For this case, electrical position before/after a switch seems not v
Regarding Fred Waechter comments on table 1 and table 3 in the 1000-3-2 std.
In my limited experience there are very few products that exhibit the type
'D' waveform and pass the table 3 limits. This waveform is the result of
of a simple rectifier capacitor filter circuit which is so rich in harmo
I expect that 99% of us deal with Plugable equipment type 'A', see 950 para
1.2.5.1.
per paragraph 2.7.3, 'For PLUGGABLE EQUIPMENT TYPE A, the building insatllation
is considered as providing short circuit protection.'
Therefore use of a single fuse is ok on smaller equipment using non-industrial
I believe the original intent of this requirement was to protect the totally
technically clueless from harm. The test finger is used simply since it is
available to anyone doing safety evaluations. Introduction of a new tool is
really a major burden.
I have typed out the key phrases in 950 follo
The interpretation of 'continuous' is unfortunately a variable and up to
the individual test Engineer with some influence by the Test House. Last
I checked around, CSA was very rigid with zero tolerance over 240 VA, and UL
would allow up to 5 seconds.
Bob
Please remember that in the EEA the 75 watt limit for type 'D' waveforms will
be reduced to 50 watts. The proposed date is July 1, 1999. It is not probable
that there will be a 'grandfather' time period, so all product shipped after
that date must be in compliance.
Also, in Japan, JEIDA harmonic
Japan has issued harmonic limits for larger products, via JEIDA. I do not
have the document number. The title is 'Guidelines for the Suppression
of Harmonics in Appliances and General-use Equipment'. These limits are
required for JEIDA members Jan 1 1996 for product first shipped to Japan
on tha
John, I suggest you rate your product for 200+ Volts only. Regardless of the
rating of the IEC inlet, you are faced with the limits on 120v outlets. Normal
outlets (with the parallel blades) are rated at 12A, and normally protected
with a 15A breaker. Dedicated outlet (with 1 each horizontal/ver
I know of several power supply manufacturers with active power factor corrected
product either availale or very near available. These are intended for
various markets and will have varied agency approvals
Cherokee International, Tustin Cal., 714-544-6665
Todd Products, Brentwood, NY, 516-345-
The limits permitted for Class D equipment, which is the special waveshape
of current associated with a full-wave rectifier and capacitor input filter,
is given in Table 3 of IEC-1000. Table 3 provides 2 limits for each harmonic,
an absolute maximum and a power-weighted maximum based on the real i
Back in the early days, just before dirt was invented, we all used UL-498
for information technology equipment. This spelled out 24 volts peak as the
allowed limit. In these early times we also had a telephony industry operating
under their own rules--an ok situation since the industry was very t
The small off-line switch-mode power supplies are designed without any user
replaceable parts. This normally includes the input fuse. This is a sensible
practice since the fuse will interrupt only when a serious failure occurs
elsewhere, such as a shorted bridge rectifier, or shorted FET, etc. W
Just several hundred mA of dc current through the chest area is lethal, so
please take some care when measuring body impedance. It may be just a tale,
but it is told that a physics professor carefully inserted hypodermic needles
in each wrist, then applied a 'traditional' VOM using a 9v transitor
I recommend the blade style auto fuse, such as LittleFuse. These are diecast
parts from a zinc alloy without welds/brazes to fracture or fatigue. The
reliability is much improved compared to the ceramic tube fuse with external
elements. The blades fit quite snugly into the slip connectors, which
regarding power line harmonics and switched vs linear---
The majority of linear power supplies are TR-packs followed by series
regulators. TR is Transformer-Rectifier, which is a line-frequency
transformer driving a capacitor-input filter. If the transformer were ideal
then the primary current w
I have a question regarding JEIDA harmonic requirements. When the product
in question has real input power less than 75 Watts, what next? Is it
necessary to claim compliance via a product lable in Kanji? Or is it ok to
just ship the product?
from Bob Den Bleyker at IBM-RTP
For Australia/New Zealand, check that your power system complies with
AS/NZS 3260:1993, Appendix 3, DC Component from AC Equipment
basically this limits the dc component of the input current to 5 mA or less,
with the goal to reduce corrosion at earthing electrodes.
Many power supplies use small i
I agree with 99.9% of the recent note from Rich Nute regarding the competancy
and morals of CENELEC BT.
I disagree with a technical point. Rich states that his products are less than
75 watts and therefore Class A. This is not correct
The exact words are...
"For Class D equipment, the limi
from--bo...@vnet.ibm.com
Several years ago I tested MOV devices from several manufacturers. All had a
nominal rating of 70 volts and 10 joules. My testing forced breakover with a
10-ohm limiting resistor. Source voltage was 120 mains and simple fill-wave
rectifier. The results were quite inter
In my opinion, any device that shifts from a high impedance to a low
impedance to protect an attached, parallel load, needs another part.
The additional part is a series energy limiter, examples a PTC device, an
inductor, a resistor, a fuse, etc. The resulting circuit is nothing more
than a shunt
plane
I don't think galvanized materials are very good for a uniform conductive
surface. Zinc forms a 'self-protective' oxide barrier which is terrific
for protection against the elements. Unfortunately the oxide thickness is
somewhat variable which results in a very variable surface resistivit
hm. Distance between SELV and Earth? This doesn't compute, per IEC-950
para 2.3 SELV is produced, Method 3, via 'adequate Earthing'. I would try
another test house.
and a general comment to all. PLEASE include instructions for use of clever
data transmission techniques for those of us who a
In my personal experience looking at just the 3d harmonic is going to get you
in trouble. Please take the time to look all the way out to the 39th and
be prepared for some unpleasent news. You might be able to create an
arguement that the current wave shape falls out of the 'special D' with
suffi
I found a section of Japan MITI-JEIDA documentation that may be of interest.
This is probably not the latest information but provides an indication of the
thought process. Document is MITI, Division 6, No. 378, dated Oct. 3, 1994.
Author is Seija Murata, Director of Public Utilities Department.
The USA ring voltage is 66v RMS, as limited by FCC. This is a 54v dc level
with superimposed 108v peak-peak ringer at approximately 20 Hz. This is lethal
level to any human in a 'low-impedance' state. Low-impedance is the result
of skin-breaks, broken lesions, immersion with saline solution etc.
Power Switch has an active power factor corrected 300 watt supply in
traditional cube shape with fan. Contact is Danice Donito at 201-478-0300.
Location is Lodi, NJ. I have tested to harmonic requirements of 1000-3-2
and it seems ok.
bo...@vnet.ibm.com
Several years ago, Dr. George Pearsall of Duke University, Durham NC
offered an excellent seminar. I took this course and do recommend it.
per the Duke website, g...@eng.duke.edu is present address.
bobdb at vnet.ibm.com
Just a few precautions---
Leakage current values must be measured per the appropriate standard. Ref
IEC-950 Annex D and G. An impedance is placed in series with the current
path simulating an adult human body, with bypass capacitors to emphasize
human sensitivity to mid-range frequencies. Simpl
Regarding harmonic content and THD over 120% for switching power supply
currents. Please let us not forget that the 'linear' power supplies of
yesteryear were series regulators fed by TR packs. And the TR part is a
transformer driving a full-wave bridge followed by a capacitor input filter.
THD
1. First a comment. The Duetschesbundepost is the only user to my limited
knowledge using 60v nominal battery voltage. I also understood that there
was a planned conversion to 48 nominal.
This could be important since fans rated at -48 are normally intended for the
maximum possible with fully c
In all of the discussions of hair-dryer in a tub scenario, no one has discussed
the GFI and how it works. It senses the current in both power leads, and if
they are not equal, it trips OFF. Thus it assumes that any current flowing
between phase/neutral is good, any current in the phase lead not r
In my opinion, the designer is responsible for the PC board temperature as
well as the semiconductor. The PC board is both a structural element of the
product maintaining creepage/clearance distances, and an insulator. PC board
failure can easily result in a safety problem. Overtemperature will
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