Brian,
I appreciate for your purposes the SPC component of
choice is the MOV. Quite right too, if you are considering
cost and device safety standards like UL 1449. SPC
standards, which tend to be for component electrical and
environmental performance, can only overlap the requirements
of an Equipment/Device safety standard.
Your perception of other protection technologies needs
revision.
True, PN junction zener breakdown diodes have a very soft
clamping characteristic. NB The zener breakdown effect only
occurs below about 7 V.
To counter the poor zener clamping characteristic a
multi-junction silicon semiconductor component using the
punch-through effect can be used. This type of technology
provides a sharp clamping characteristic from a few volts
upwards. Punch-through voltage limiters can be found in
Ethernet ports. These SPCs protect the Ethernet PHY chip
against damaging overvoltages (even though 5-volts may not
sound like an overvoltage)
Above 7 V a different breakdown effect comes in -- the
avalanche effect.
Avalanche breakdown diodes have a good sharp clamping
characteristic. You may see them referred to as SADs,
Silicon Avalanche Diodes. The three-letter acronym powers
that be in the IEC said they didn't want a sad component and
so the ABD, Avalanche Breakdown Diode, acronym came into
being. Unfortunately, many people, who don't know the
difference between zener and avalanche breakdown, call ABDs
zeners, which is totally wrong.
All PN junction semiconductors have relatively low thermal
capacity and energy absorption capability. These
deficiencies can be countered by using series and parallel
combinations of ABDs. There are several companies in the US
making AC Power SPDs using this approach -- costly, but
these have a better clamping performance than an MOV.
The GDT uses gaseous discharge and the switching time from
sparkover to the low-voltage arc can be in the tens of
nanoseconds. The problem is that the sparkover voltage is
dependent on the rate of voltage rise. This overshoot of
surge sparkover to AC sparkover can be something like 2:1.
This is not a firm ratio but dependent on the AC/DC
sparkover voltage. An 80 V GDT will have a higher surge
sparkover voltage than the "sweet spot" 250 V GDT.
An interesting trend I've noticed is for GDT MOV series
combinations where PLC is being used. Put MOV protection
(read capacitance) on the AC supply and PLC reach and
environmental pollution are reduced. Protect using a low
capacitance GDT MOV series combination and PLC reach and
environmental pollution are maximised.
Regards
Mick
On 14/11/2011 21:04, Brian Oconnell wrote:
o further abuse a meme - moar standards! [insert troll-face here]
Another member has previously commented that there a several type of
components used to arrest a surge. For the purpose of my OP, was focused on
a MOV-type SPD as defined under UL CCNs VZCA2 and VZCA8, where the effective
standards are UL1449 and CSA C233.1, and various CSA TILs. And of course,
the wondrous IEC61051-2. This is another case where EMC requirements
(61000-4-5,6) can affect product safety. Last year's update to the 2d ed of
60950-1 for my component power supplies was, for some models, quite an
adventure.
A problem NOT ADDRESSED by TC108 is the increased energy AFTER a surge, or
during a SFC, due to the 120% rating requirement. Perhaps this was a
principal intent of UL1449 3d ed - verify that the high E and I do not make
the MOV puke it guts and start a fire. There are many sources of increased
energy - my two problem children are the effects of an inductive kick to the
circuit being 'protected' by the SPD*after* the current interrupt device
has opened; and the higher voltage (CV^2), during a surge, at which the SPD
will start conducting.
As for GDTs - they take longer to get to low Z. And zener-type arrestors do
not have a sharp knee at the conduction level. The ZnO MOV seems to be the
best chance of survival, assuming the other circuit components can handle
much higher coulombs running around before clamping.
Moar standards! Moar unintended effects! Moarrrrrrr....
Brian
-----Original Message-----
From:emc-p...@ieee.org [mailto:emc-p...@ieee.org]On Behalf Of
ralph.mcdiar...@schneider-electric.com
Sent: Monday, November 14, 2011 11:31 AM
To:EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] MOV requirements
Does the industry really need another standard, I wonder?
Ralph McDiarmid | Schneider Electric | Renewable Energies Business |
CANADA | Regulatory Compliance Engineering
From: John Woodgate<j...@jmwa.demon.co.uk>
To:EMC-PSTC@LISTSERV.IEEE.ORG
Date: 11/11/2011 06:48 AM
Subject: Re: [PSES] MOV requirements
In message<4ebd2d05.5050...@ieee.org>, dated Fri, 11 Nov 2011, Mick
Maytum<m.j.may...@ieee.org> writes:
>MOV, Varistor, VDR; Metal-Oxide Varistor are all names for a
>voltage-limiting component using a particular technology.
I think this 'particular technology' is an important point. As I
understand it, devices like Transorbs (TM?) are quite different from
MOVs, being avalanche diodes with a very high thermal capacity. They
seem to be rarely used (cost?) but have desirable characteristics (like
not exploding!). Are there relevant standards?
--
OOO - Own Opinions Only. Trywww.jmwa.demon.co.uk andwww.isce.org.uk
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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