I have no idea regarding electrical stuff so please help me!
I wish to protect my computer from electrical damage since last week
some short circuit in my house fried my mother board...
Now I have upgraded my system to a P4 3.4Ghz (runs on a 500 watt SMPS)
and wish to protect it with an independent MCB. Please suggest a proper
MCB to use. what rating of MCB shall i use.
It would help if you states your location. Voltages and rules vary.
In general what you need to think about is getting a UPS. A decent
online UPS will stop most things happening on the supply from damaging
your computer. Unless you use a laser printer, a 1000W one will do nicely.
In the UK for example, which is where you probably aren't, an MCB
protects a ring of wiring and outlets, not usually a single one into
which a desktop computer is plugged. The MBC protects the ring of wiring
- not the individual units connected to it, each of which has its own
(lower rated) fuse. Y electrical system MV
I would question the fact that a short circuit in the house would damage
your circuit board. Was the damage coincident with this short circuit?,
there may have been transients generated upon the SC; but these would
probably have been dampened at the power supply unit for the computer. Quick
Blow fuses are usually installed on the computer power supply to reduce the
effects of surges and before any MCB would operate, (even if the fuse
ruptured, the damage may not have been prevented).
On Thu, 2 Mar 2006 09:47:50 +0000 (UTC), "John McLean"
The most obvious (read typical) failure is the Power Supply itself.
Upon failure, however, it may have posed a serious low impedance to
the AC line, causing a breaker to blow.
The new PC can be hooked up on that same circuit. His house wiring
is likely just fine.
The new 550 Watt PC supply does NOT consume 550 W. It would ONLY do
so if it were fully loaded. The 550 Watt declaration is an indication
of the supply's maximum capacity, not what it consumes. At its
maximum capacity, it will actually consume more than 550 Watts.
His best protection is to go ahead and spend the bucks to get a good
750 VA or more UPS for the unit.
If his monitor is a flat panel device, he can plug both it and the
computer into the UPS. If he has a CRT, he should (though not
required) plug the CRT monitor into a separate outlet, not the UPS
outlet(s). Most modern UPSs have some line conditioning in them, and
IIRC they all stop surges, spikes, and brown sags/outages. If the UPS
he gets is powerful enough he can plug the PC, monitor, and printer
into the thing.
I just love how all these modern appliances use less and less energy
to do the same job (note I didn't say work) as the old gear of the
Still, it makes one wonder which three books George took back to the
Eloi and Weena... hehehehe don't let that one catch you off guard!
Given that you say you had a short circuit in the house, I would suggest you
invest in a good surge protected socket outlet unit.
Have a look at the Belkin SurgeMaster Gold Fg723uk3M-f-GY with unlimited
cover if your equipment is exposed to external surges,
they will protect you PC and its accessories from this type of damage, see:
They also provide protection from surges in your telephone lines damaging
your modem or computer.
Wow. So many in alt.electical.engineering that don't even have basic
electrical knowledge. The only responsible reply so far is from John
Let's confront some of those myths. The UPS. It typically connects
your computer directly to AC mains when not in battery backup mode.
Worse, still, it can even contribute to motherboard damage by giving
destructive voltages a path around computer's power supply.
Same applies to a myth based recommendation for a Belkin power strip
protector. Does that author even know what a plug-in power strip
protector does? Obviously not.
A power supply properly designed will not damage a motherboard.
Short circuit damaged a motherboard? Not likely. A short circuit
would trip the circuit breaker. What circuit breaker tripped? If that
short circuit caused 240 volts on 120 volt line, then the power supply
would be damaged - not power supply.
Short circuit from AC to a peripheral line? A short into printer
cable or serial port cable? That would damage part of a motherboard.
So why were two insulated wires both exposed AND then permitted to
touch each other?
Instead of assuming short circuit, instead back up. What exactly was
damaged? Did someone just start wildly replace parts - a motherboard -
and suddenly the computer is working? Maybe that motherboard was
intermittently shorting to the case? Moving a perfectly good
motherboard would have fixed what only appeared to be a damaged
motherboard. IOW details should include what you know and not what
someone has speculated.
Meanwhile, a MCB would not protect a motherboard. MCB is for
protecting humans - after transistor damage has occurred.
Ignore the UPS and power strip recommendations. They are not based
in knowledge of what those devices really do. Instead, first learn why
or even if you really had motherboard damage. Post known facts instead
of conclusion to better avoid future damage.
An online ups does not pass through the mains, neither do many line
interactive models. Also, how does it bypass the power supply and supply
destructive voltages to a motherboard? I have never seen a UPS that
required you to wire its output directly to the motherboard. Of course we
have only tested a few hundred of them in our lab. Perhaps we missed that
I think you are the one who should learn a little before making
I didn't know if he was referring to the RS232/USB computer link from
the UPS - but, IME, they are usually optically isolated. Or whether he
meant that a UPS could send destuctive voltages to, say, the monitor and
hence feed them back through the 15 way Dsub to the mobo. But then I
haven't seen that many either, I just design them.
I wondered if he thought they came in strawberry flavour, or ribbed or
Are you asking how a protector located too close to electronics and
too far from earth ground will shunt a transient into motherboard?
Those front end MOVs simply put the transient on all AC wires. One
wire bypasses power supply and connects the transient direct to
Meanwhile USB standard is direct wire connections - not opto
isolated. Is it opt isolated in UPS? Does not matter because a
destructive transient has already been shunted directly to motherboard
on ground wire - bypassing both UPS and computer power supply.
Effective protection at a computer is already inside its power
supply. Will a line interactive UPS protect the computer? Totally
irrelevant. Anything that UPS is going to block is already inside the
Effective protectors must be earthed. That has been standard even
before WWII. Even before MOVs existed. Even before transistors
existed. The effective protector must earth before destructive
transients can enter a building. Earth so that destructive transients
do no overwhelm protection already inside appliances.
Meanwhile those plug-in protectors don't even claim to provide the
protection that so may assume. Don't believe me? Then post the
numerical technical specifications from that manufacturer.
An example. APC once posted this to claim UPS protection:
Today they do not even provide that much. In fact one APC UPS only
claimed to protect phone line and only listed numbers of joules.
Notice the glaring omission. It claimed no power line protection (why
did they stop doing that when the circuit did not change). They will
not discuss the type of transient that typically causes damage -
longitudinal mode. Now they will not even discuss Normal mode. Why?
They hope we will assume that normal mode protection is protection
from all types of transients.
Those plug-in protectors - power strip or UPS - do not provide
effective protection. A most glaring reason why is the missing earth
Meanwhile MCB - a circuit breaker - will not protect a motherboard.
For what its worth, I think w_tom's argument is that if there is a
common mode surge it will be shunted to the ground wire. The impedance
of that wire will cause the ground potential at the surge protector to
rise relative to the ground at the panel. The ground at the panel is the
common reference potential for the telephone lines and maybe wiring that
connects from other external devices (LAN, etc) to the computer. The
surge protector and thus computer ground will then be at a significant
difference from the phone wires (and LAN...) which can destroy parts.
'The ground wire connects the transient to the computer.' He wants
grounds (for earthing purposes) to only be connected by short wires to a
If the computer is connected only to devices that are connected to the
same surge protector the rise of the local ground doesn't matter since
the ground and power wire voltages are clamped with respect to each
other. If there is wiring from phone, LAN they would be protected if
connected through jacks on the same surge protector that clamps their
voltage to the ground/reference at the surge protector. IMHO clamping
all the voltages to a common ground/reference is more important than
Protection is already inside computer power supplies? They don't have
common mode MOVs that would do the same thing?
Agreed. If you are protecting a multiport device, such as a computer with
LAN and/or modem connections, you MUST use a multiport surge protector. As
you say, the earthing (or grounding) is not what is important. What is
important is that all of the "ports" rise and fall at the same potential.
This applies to other devices like televisions also.
Another, more expensive, option that is often done in industrial
environments is to use optic isolation on all comm ports before they enter
the computer. Again you are ensuring that all ports rise and fall together.
Charles Perry P.E.
In the example I had provided, with an adjacent protector, that is
exactly what happened. All adjacent item's voltage rose to the same
common potential causing currents to flow in destructive directions.
One most common path is through modem. Another is down a mouse wire
that is draped on floor, baseboard heat, or whatever. This is why
telephone switching stations don't put their protectors adjacent to
electronics. This is why Polyphaser - an industry benchmark -
discusses this in their example of a damaged telephone facility. From
memory, I believe that application note is:
Stated was that all appliance already have internal protection as was
even required as part of CBEMA standards over 30 years ago. You have
erroneously assumed that protection is provided by MOVs. Well
manufacturers once installed MOVs in equipment. Find them even in the
Apple II. But MOVs at the power cord are too far from earth; are not
effective. Appliances have internal protection ... without MOVs.
Return to what Ben Franklin demonstrated in 1752. What made the
lightning rod effective? Earthing. A lightning rod is only as
effective as its earth ground. Too many waste time arguing over
'pointed verse blunt' rods when the most critical aspect is the
quality of and connection to earth ground. Same applies to effective
protection. Same is why plug-in protectors - UPS and power strip -
avoid all discussion about earthing. No earth ground means no
effective protection. An MOV inside an appliance is not effective -
all but no earth ground.
Protection is about earthing destructive transients before transients
get to equipment. Commerical broadcast electronics atop the Empire
State Building suffer 25 direct strikes annually without damage. The
concept were proven by GE and Westinghouse papers even in the 1930 -
the technology being that old and that well proven.
Wire has impedance. That means a connection to earth ground must be
short ('less than 10 feet'), no sharp bends, not inside conduit, no
splices, and not bundled with other non-earthing wires. Yes, impedance
to earth is essential to effective protection. This is what plug-in
protector manufacturers hope you never learn.
So what do MOVs on the power cord or inside the appliance do? That
wire to earth is bundled with all other wires - creating induced
transients. It has sharp bends in each junction box all the way back
to breaker box. It has numerous splices. IOW MOVs inside an appliance
or inside a plug-in protector has no effective earthing. No earth
ground means no effective protection. This is exactly what plug-in
protectors hope you never realize to sell their ineffective products.
Appliance protector is only as good as its earth ground. No way
around that fact.
Most UPSes are not on-line type that cost hundreds of dollars. Most
UPSes connect a computer directly to AC mains when not in battery
backup mode. Another may have also mentioned a line interactive UPS.
But it does not provide nor claim the protection he claimed nor is it
relevant to what was posted.
An incoming transient is shunted to all AC electric wires. Which
wire bypasses both UPS and computer power supply to connect directly to
computer motherboard?. That same wire that a plug-in UPS dumps the
destructive transient onto. The adjacent plug-in protector has simply
connected a transient directly to motherboard - bypassing computer
Furthermore, if the on-line UPS does stop or block a destructive
transient, then computer's power supply has already accomplished that
task. Again a UPS accomplishes nothing.
Two examples. One demonstrates how plug-in protectors - UPS or power
strip - can even connect transients directly into an adjacent computer.
I have even traced such damage. Replacing ICs in a path created when
adjacent plug-in protectors shunted a transient into destructively into
the adjacent comptuer and then through other computers on its network.
This is also why telephone switching stations prefer protectors up to
50 meters distant from the computer - and close to earth ground. The
original poster asked for protection for his computer. Neither the UPS
nor power strip manufacturer even claims to provide protection that
those other posters recommended. But then, as demonstrated by example
and why that damage happened, plug-in protectors can even contribute to
damage of adjacent electronics. I have even traced out such failure
created by adjacent plug-in protector through a powered off computer.
Charles, unlike the other posters who would recommend ineffective and
grossly overpriced protectors, I have long had respect for your posts.
But Roy L Fuchs posted in gross error and demonstrated his technical
grasp by only replying with insults. I have too much respect for our
previous exchanges to acknowledge such a response from you. If it
provides protection that even the manufacturer does not claim, then
please explain why you think otherwise. Belkin does not claim the
protection being posted here. For it they did, then they would list it
with numbers for each type transient.
If you think otherwise, then explain how a Belkin protector earths
the typically destructive common mode transient? Its manufacturer does
not make such claims. Protection that would work at the computer is
already inside that computer power supply. Essential to protection is
not blocking transients. Computer's power supply already does that.
Essential to making a protector effective is shunting to earth ground.
That Belkin has no effective earthing - is ineffective. And does not
even claim such protection. Plug-in UPSes contain same protector
Where does the 750 VA plug-in UPS recommended by Roy Fuchs provide or
claim to provide effective comptuer hardware protection? Meanwhile the
MCB will do nothing useful for motherboard protection.
You do agree?
Charles Perry wrote:
Ferro output UPS are considered Line Interactive. Nothing like a big slug
of iron to help with transients.
See my reply to Bud, and my first post to this thread. A multiport surge
protector will help since it ensures that all ports (comm and power) rise
and fall at the same time during a transient. There is no magic in "earth",
effective surge protection is more about bonding and potential equalization
than trying to shunt things to earth.
Charles Perry P.E.
The ferro unit is a big slug accomplishing what is already inside a
computer power supply. Meanwhile, completely bypassing that 'big slug'
is a safety ground wire that carries a destructive transient around the
'big slug' and around the power supply, directly into motherboard.
As I noted in the previous example, that plug-in protector only
shunted (connected, transferred) a destructive transient to all other
wires. Now that transient is on all other wires and still seeking
earth ground. It destroyed the powered off computer AND found earth
ground via other networked computers. We traced the destructive path
by replacing damaged ICs.
Again, this is why telephone exchanges, connected to overhead wires
everywhere in town, do not use adjacent protectors. The protectors are
connects as short as possible to earth ground AND best located 50
meters from a switching computer. Same applies to household
electronics protection which is why all incoming utilities must enter
at a common point so as to connect to a single point earth ground.
What is THE most critical component of transistor protection?
Earthing. What do plug-in protectors not provide a connection to NOR
do they discuss? Earthing. Look at he specs for that UPS or power
strip protector? Where do they cite protection from each type of
transient? They don't because they don't provide effective protection.
The 'big slug' ferro-reasonant unit is supplementary protection. But
without a properly earthed 'whole house' protector, even the ferro unit
is compromised. The ferro unit can only be one component in a
protection 'system'. The one and most critical 'system' component is
earthing. No earth ground means no effective protection. Effective
protectors are part of a 'system' that includes the most critical
component - earthing and how bonding to that single point is installed.
Earthing is far more critical than any protector. There is no magic
in protectors. Some incoming utility wires are protected with no
protectors - coaxial cable. But again, that protection is only as
effective as its earth ground. Earthing is the one 'system' component
that protection 'systems' must have.
Charles Perry wrote:
Earthing is not that important. Potential equalization is. Think aircraft,
spacecraft. If all of the ports rise and fall together, no damage is done.
The problem is when the grounding conductor of the power source increases in
potential, which it will ALWAYS do (no zero impedence wire in most homes),
and other ports, such as telcom, cable, or network, do NOT. You now have a
dangerous potential across some sensitive component(s). A good surge
protector ties all of these systems together so that they must rise and fall
I suggest you try to find some publications by F. Martzloff previously of
NIST. He did a lot of research in this area and produced some very good
publications on surge/transient protection of electronic equipment. Much of
his research was done in our lab. This webpage has a nice bibliography of
some of his work:
This is a good overview of surge protection produced for NIST.
Particularly look at page 18 about multiport devices. The last page
references a document that used to be available on our old website. Email
me backchannel and I can provide a copy.
BTW, the PEAC mentioned on those pages is the lab I now manage.
Charles Perry P.E.
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