Richard,
There is a study, ANSI C62.41, that somewhat addresses your interest.
In my knowledge, there is no universal standard or published work
that details the magnitude, joule content, or frequency of occurrence
of transients since there are many sources and conditions vary widely
depending upon location. Here in Arizona we get a thunderstorm just
about every day at this time of year, while in the UK they are
somewhat less frequent. The ANSI study cites a typical lightening
stroke induced transient as a 1.2 by 50 microsecond pulse. The wave
front rises to 90% of peak value in 1.2 microseconds and decays to 50%
of peak value in 50 microseconds (48.8 microseconds after the 90%
peak). There are other studies and I have read one that contends that
the lightening stroke is a 50 by 300 microsecond pulse. The
OVERVOLTAGE INSTALLATION CATEGORY referred to in IEC 664 AND IEC 1010
are differentiated by the source impedance driving the pulse. For a
CAT III location, IEC 1010 (Annex D.10) requires a 2 ohm source to
generate the pulse with a peak value as called out in Annex J.
Haefely-Trench make a surge generator that meets these requirements.
It charges up a 9 micro-Farad capacitor to any voltage between 2.5 kV
and 30 kV. (At 30 kV that's 4000 J !) This is a 6 foot tall cabinet
that weighs around 800 pounds , so not something you are likely to
kludge together from spare parts. The capacitor is discharged through
a pulse shaping network and the source impedance is the open circuit
voltage divided by the short circuit current. For a CAT II circuit,
the source impedance requirement is 12 ohms. With the Haefely, this is
accomplished by putting a large 10 ohm resistor in series with the
generator.
Hope this helps.
Dan Teninty PE
Hughes Missile Systems
Tucson Arizona
______________________________ Reply Separator _________________________________
Subject: Data on mains transients requested
Author: [email protected] at LMSMIME
List-Post: [email protected]
Date: 7/24/97 6:52 AM
Dear Safety & EMC folks,
I am interested in obtaining data on the number, duration and amplitude of
mains transients at different parts of the mains distribution network. It
would be great if someone could point me to information published by the
IEE, IEEE or whoever.
IEC 664-1 classifies different parts of the mains distribution network as
being Installation Category II, III, IV etc. IEC 950 restates IEC 664-1
in saying that equipment plugged into a normal socket outlet in an office
or home is Installation Category II: hence the clearances in Clause 2.9.2
of IEC 950 are based on 2.5 kV transients for a 230 V supply.
I am also aware that various EMC (well, immunity) standards require
equipment to be *hit* with transients, but not being an EMC engineer I
cannot quote standard numbers or values at you.
It is one thing knowing the assumed maximum transient voltage, but that is
not sufficient to work out the amount of energy to be absorbed by an MOV or
other such device during its working life. I have seen MOVs of various
sizes used in power supplies, but just how big is big enough I don*t know
(but I*d like to find out).
For those of you that don*t keep up with standards let me say that the
draft 3rd edition of IEC 950 has an Annex that provides an alternative
method for determining minimum clearance distances that includes a test for
injecting the assumed transient at the mains input and then measuring what
the resultant transient at the insulation under consideration.
Unfortunately there is also a requirement that "surge suppressors in
PRIMARY CIRCUITS are disconnected". This restriction may be due to the
nasty habits that MOVs can get as they get older (i.e hot and *leaky*) -
but then maybe devices that exhibit this behavior were not adequately rated
in the first place?!
Information and constructive comments please.
Thanks (I hope!)
Richard Hughes
Nortel plc (UK)
(By the way, this is all the signature I think is needed, but no doubt
someone will *correct* me).
