Technology Connections doesn’t get nearly enough love here
Circuit breaker is only “emergency backup protection”. A warning that a human has screwed up or some hardware issue exists. Breaker should never be the device that protects from an overload. A human must do that. If two devices overload a circuit, then both devices (as made obvious by amp numbers on each appliance’s nameplate) must not be connected to that circuit.
Protector is only for spikes that occur in microseconds. Spikes that are only thousands joules are made irrelevant by what is inside appliances. Spikes that are a concern can be hundreds of thousands of joules. How many joules will destroy that plug-in protector? Thousands. That plug-in (Type 3) protector is an appliance that needs protection - by something completely different that requires a separate discussion.
Circuit breaker is about a constant flow of current - in amps. Not a microsecond spike that is measured in joules.
Wall receptacle can provide up to 15 amps. Any plug, that can mate to a wall receptacle, is connected to an appliance that will consumer less than 15 amps. But connecting two such plugs into the same wall receptacle (using a power strip) compromises that human safety feature.
That power strip must always have a 15 amp circuit breaker. And again, as “emergency backup protection” should tell a human he has made a mistake. Should not be depended on to always disconnect from an overloaded circuit.
Nitpick here (sorry!), but an MOV doesn’t create a dead short when the voltage between its terminals exceeds its threshold voltage. An MOV is a clamping device; it tries to maintain a constant voltage when it conducts, e.g. a voltage of 200 V. Same with a solid-state TVS. A gas discharge tube, OTOH, is a crowbar device; it (sorta) creates a dead short when it conducts. I say “sorta” because there’s a voltage of around 10 V to 20 V across the GDT when it conducts, not 0 V.
Thanks - I think all that makes sense. If I understand correctly, the MOV circuit is parallel to the load? Whereas a fuse or circuit breaker would be in series.
That is correct.
MOVs look like this:
Here is a power strip with a single MOV, which, while not very good, is enough for the power strip to be labeled as a “surge suppressor”. Note that the MOV is connected between the black and white wires, which are the hot and neutral conductors.
That’s correct.
A circuit breaker is always “looking at” the current in a circuit. If it sees the current go too high, it opens, thus making the current zero everywhere in the circuit. With no current, the voltage drop anywhere in the circuit is also zero, and everything is safe. A circuit breaker in your home’s breaker panel is a mandatory device, but it is only looking at current to protect the wiring in the walls of your house; not the load, and not you.
An MOV is an optional device used to squelch voltage transients. Normally, the voltage at your outlet is a nice, pretty sine wave with peaks at +170 V and -170 V. But every-now-and-then the voltage will spike up to (for example) 500 V for a fraction of a second. The circuit breaker in the panel couldn’t care less about this brief voltage spike, but whatever is plugged into the outlet might care. And it might be damaged due to it. So an MOV is placed in parallel with the load. It is normally an open circuit. But when the MOV sees the voltage spike up, it instantly “turns on” and maintains a constant voltage. (It can do this because the transformer on the utility pole, and all the wiring in the house, have some impedance. If this “source impedance” were zero, an MOV would be ineffective.) The energy is dissipated by the MOV in the form of heat. When the voltage spiker is over, the MOV goes back to being an open circuit.
The above is an over-simplification, but I think you get the drift.
never mind
Usually because you already have the surge protector there, and it’s easier than getting access to the wall socket, which may be awkwardly placed around furniture. In fact, I think most of the surge protectors I own are more for making plugging things in more convenient than they are for actual surge protecting.
It’s also easier to reset the circuit breaker in a surge protector than it is to have to go out into the garage (especially in winter) to mess with the main circuit breaker. Not that you should intentionally try to trip it, of course.
Yeah, it’s generally a bad idea because your average layman doesn’t understand electricity. Heaters generally can have a fairly high wattage rating, and most people have fairly flimsy extension cords in the home (like those 2-wire white or brown cords you buy at Ace Hardware) that are adequate for Christmas lights, but not much else. I run a space heater in my RV when the temperature gets below freezing, but my extension cord is a heavy gauge cord that is designed for use with things like table saws and the like and can handle the load.
My computer, monitor, and gateway are plugged into a surge protector outlet strip, and they blow circuits out every time I open certain Pit threads.
My breakers trip when the schadenfreude gets too high.
If it’s really that hard to turn off the space heater before you turn on the coffee pot (if it’s a question of remembering to do it, put a note on the coffee pot), the thing that needs to be on another line is the coffee pot. It probably is safe on an extension cord, so run one in from another room on a different circuit from the space heater, and use that. Although, if it is not an ordinary home-use coffee maker, read the directions before you use an extension cord.
FWIW, while space heaters do always say don’t use an extension cord, they say this because most people do not know how to check the power rating of an extension cord, and will start fires. If you do know how to check, you probably also know that it is possible to use one safely (there are other precautions). I was a power generator mech/maint tech in the army. It is still not advisable to use an extension cord, if it isn’t necessary, but there you are.
If I understand correctly, the MOV circuit is parallel to the load?
That does not do protection. Because a surge is a current, everywhere inside house, hunting for earth ground - destructively. An adjacent protector simply gives it more paths to find earth, destructively, via that or any other nearby appliance.
An IEEE brochure bluntly demonstrated this. A protector in one room was only connected to safety ground (not earth ground). So it earthed a surge 8,000 volts destructively through a TV in an adjacent room. Where was protection?
Unfortunately, many fail to learn relevant currents. Destructive currents can be longitudinal mode. That means a surge is incoming to an appliance only any one or on all AC wires. Many only used wild speculation that a surge is incoming on one AC wire and outgoing on another AC wire. That would be a metallic mode current - which is not a surge.
Effective protection answers this question. Where do hundreds of thousands of joules harmlessly dissipate. View that above protector as an example of a an extremely dangerous protector. Its near zero (hundreds) joules are why protectors fail catastrophically. (MOV manufacturers are blunt; MOVs must never fail catastrophically.)
Worse, an essential thermal fuse does not exist.
When protectors are grossly undersized, then a thermal fuse must disconnect protectors part, as fast as possible. And leave (connects) that surge into all appliances. No problem. Protection inside appliances is superior. As previously posted numbers demonstrate. Electronics will routinely convert that surge into low DC voltages that safely power semiconductors.
That protector demonstrated why no plug-in (Type 3) protectors should be used if a ‘whole house’ solution (Type 1 or Type 2 protector) is not properly earthed. Only earth ground harmlessly dissipates hundreds of thousands of joules. Effective protection only exists when a surge is not anywhere inside a house.
Only protectors that are effective connect hundreds of thousands of joules to earthing electrodes. Every facility that cannot have damage (today and over 100 years ago) only does that. A plug-in protector (Type 3) must remain more than 30 feet from the main breaker box and earth ground. That protector picture demonstrates why is must be far away to do much less protection (therefore not create a fire).
Fire codes also say an extension cord is only for temporary services (ie 30 days). Overloading is not why extensions cords (powering space heaters or anything else) are dangerous. Physical insult (ie motion) are the most common reason for extension cord created fires. So many such fires are why arc fault breakers were created. Wires inside walls do not suffer this insult; are safe.
Why do some appliances (ie coffee makers, toasters) have short power cords? To reduce what creates a danger; insults to its power cord.
I stopped reading when I read this. Are you saying a surge starts as a current? Through what conductor(s), precisely? Where is the current coming from? Are you saying surge current is being “pushed” through loads?
I am certainly not an expert in this field, but it’s my understanding that “spikes” or “surges” usually occur because the voltages at the secondary of the 120 V / 240 V transformer are higher than normal for a brief amount of time. These voltage spikes or surges can cause a couple problems to loads:
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A voltage spike or surge could cause dielectric breakdown to the wiring and/or electronic components in the load. I believe this is the most common failure mode. The failure, in turn, could cause excessive current to flow through the device, causing even more damage.
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A voltage surge (not spike) could cause the current to increase through the load due to Ohm’s law, resulting in excessive power being dissipated by the load. I believe this failure mode is more rare, since the voltage surge would have to have a long enough duration to cause the load to overheat.
In my (UK) home we have a washer and a dryer plugged into the same extension lead. If both are on at the same time, and the heater in the washer kicks in, the 13 amp fuse in the lead blows. The breaker at the board doesn’t trip because the fuse blows first.
It’s not hard to remember to avoid having them both on at the same time.
My experience in the U.K. is that all plugs are fused, including the plug on an extension. I assume this is by regulation.
But I don’t think I’ve ever seen a fused plug in the U.S.
Are you saying a surge starts as a current ?
A surge is a ‘current source’. That means voltage will only increase as necessary so that a given current will flow. Anything that foolishly tries to ‘block’ a current source suffers an increasing voltage. Voltage increases only enough to blow through anything that foolishly tries to stop it.
Effective protectors are rated by current. For example, lightning can be 20,000 amps. So a minimal (and effective) ‘whole house’ protector is 50,000 amps. If connected low impedance (ie less than 10 feet) to properly installed earthing electrodes, then that lightning strikes creates only a tiny voltage.
If that current is connected low impedance (ie hardwire has no sharp bends or splices) to earth, then virtually no voltage is inside trying to blow through household appliances.
Surge protection is always about the independent variable: current. Voltage is a dependent variable (concept from school mathematics). That current only creates a destructive voltage when it does not have a low impedance connection to what it must connect that cloud to - distant earthborne charges. Via earthing electrodes.
If that current path is not inside, then best protection inside every appliance is not overwhelmed. Nothing new here. This is how protection was done routinely over 100 years ago. This is the principle that Franklin demonstrated over 250 years ago. This is a concept completely misunderstood by so many deceived consumers educated by advertising myths, hearsay, wild speculation, and subjective reasoning.
Protection is always about how that current source connects to earth. On a path outside that creates a tiny voltage. Or on a path that connects that same current inside; creating a massive (destructive) voltage.
A protector is only as effective as its connection to and quality of its earth ground. So that a massive current creates almost no voltage.
How much voltage must 120 volt electronics withstands without damage? International design standards many decade ago (before the IBM PC existed) required up to 600 volts. Today’s electronics are more robust. Meaning avert damage from surges (including direct lightning strikes) is routine. But only when that current connects low impedance (ie a connection that is not inside metallic conduit) to earth on a path that remains outside.
A voltage surge is a spike or a transient. A spike that is connecting a current to earth. Either harmlessly outside. Or that current is inside hunting for earth ground destructively via appliances.
For example, a transient incoming on AC mains is incoming to all household appliances. Are all damaged? Of course not. Current only flows when it is both incoming to and outgoing from that appliances - at the exact same time. For example, what is a best outgoing path from a TV? Via its coax or HDMI cable. Since that is properly earthed (has best protection required by codes and standards) before entering.
Incoming to everything via AC wires. Best outgoing path is that TV cable or HDMI port. Damage is often on the outgoing path. (Not an incoming path as many mistakenly speculate.) Then all other appliances are protected by that TV.
A surge need not find earth ground, destructively, via a dishwasher, clock radios, garage door opener, doorbell, refrigerator, central air, GFCIs, washing machine, and smoke detectors.
Since that current created a moderate voltage on its connection via that TV, then its voltage remains well below what would be destructive to other appliances. What is THE most critical item always in protection? That low impedance connection to and quality of single point earth ground. Then a massive current creates a tiny (not destructive) voltage.
Informed homeowners know that and therefore connect one ‘whole house’ protector less than 10 feet to earth ground electrodes. To protect everything. To have best protection for about $1 per protected appliance.
A protector, adjacent to an appliance, must somehow ‘block’ or ‘absorb’ a surge. How does that 2 cm protector part ‘block’ what three miles of sky cannot? Always consults numbers.
How does that thousands joules protector ‘absorb’ energy created by that surge current: ie hundreds of thousands of joules. Obviously, no plug-in magic box can. What does not foolishly try to ‘block’ a surge? What routinely and harmlessly dissipates hundreds of thousands of joules? Earth ground. All were first introduced to these proven concepts in elementary school science.
BTW fuses do nothing to protect from surges. Obvious if one learns some simply specification numbers. But again, honesty only exists when numbers apply perspective to every reason why. No numbers (subjective claim) is a first indication of deception or deceit.
Where are hundreds of thousands of joules harmlessly absorbed?
Which ought to be clear after a moment’s thought: fuses are rated in amps. Power sources don’t send amps through circuits; appliances draw them. The machine, appliance, whatever is plugged in, is called the “load.” The load determines how much power is sucked out of the source.
If you don’t mind my adding a bit of info:
When too much current gets drawn, and the circuit can’t handle it, it’s called an “overload,” not a surge. But it does blow fuses, and also trips breakers.Shorts can cause them by bypassing resistors, and just generally causing the circuit not to route the way it is supposed to. Other malfunctions can cause a part to work too hard and draw extra current, but I won’t list them all.
I will note, though, that user error is a big cause of circuit overload at the breaker. It’s entirely possible to look at what a breaker is labeled, know the load it can handle, and then add up what’s on it, to know whether or not you can add a particular appliance. Albeit, I think an electrician would advise people never to use more than 80% of a breaker’s amperage. That was the general rule in the military when we set up a generator.
Best protectors are rated in amps. Lightning is typically 20,000 amps. So a minimal ‘whole house’ protector is 50,000 amps.
But those amps are completely different and irrelevant to amps that rate a fuse.
300 consecutive surges could pass through a fuse before it even thought about tripping. Since time is also a relevant parameter. Fuses take tens of milliseconds or even an hour to trip. Surges are done in microseconds.
Fuse also has a voltage rating. If voltage exceeds its number (typically 250 volts), then a blown fuse can continue conducting amps.
All reasons why fuses do not protect from the relevant anomaly.
Those examples are accurate - but subjective. It applies to everything in life - not just electronics. Perspective (ie numbers) are also necessary to know something.
Even the word “surge” is subjective and deceptive until defined by numbers.