which MCB rating to use for my computer

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).
jaymack

On Thu, 2 Mar 2006 09:47:50 +0000 (UTC), "John McLean" Gave us:
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 past.
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.
Best regards
BillB

Wow. So many in alt.electical.engineering that don't even have basic electrical knowledge. The only responsible reply so far is from John Mclean.
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.
snipped-for-privacy@gmail.com wrote:

LOL. An online UPS, as I suggested in my post, does not "typically connects your computer directly to AC mains when not in battery backup mode."

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 one.
I think you are the one who should learn a little before making recommendations.
Obviously you do not.
Charles Perry P.E.

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 something?

On Thu, 02 Mar 2006 23:26:09 +0000, Palindr?me Gave us:
Yeah... the top posting retard also said that we don't know anything electrical. IIRC, aren't you that engineer I wanted to marry? :-]

On Fri, 03 Mar 2006 01:23:59 +0000, Palindr?me Gave us:
If the UPS is five years old or less, they are ALL optically isolated to approx. 1kV. Perhaps even 2kV.
Heheheheh... The switcher switches the switched (his brain). I'd rather have a bottle in front of me than a frontal lobotomy.
Good job! Hahahahahaaha!

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 power supply.
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 circuits.
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:

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 motherboard.
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 computer.
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 ground connection.
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 ground rod.
My comment 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 earthing.
Protection is already inside computer power supplies? They don't have common mode MOVs that would do the same thing?
bud--

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.

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.

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.
Bud-- wrote:

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 together.
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: is a good overview of surge protection produced for NIST. 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.

On 3 Mar 2006 18:00:00 -0800, "w_tom" Gave us:
snip
Top posting Usenet RETARD!
Do you know ANYTHING about ANYTHING?