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Author Topic: We LOVE MOVs, but what about...  (Read 2702 times)

frank kayser

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We LOVE MOVs, but what about...
« on: October 21, 2014, 01:18:09 pm »

Hi,
I've read just about every post about MOVs and the risk/reward of having them in a powerstrip. 


I have my rack mounted switched "power strip" open to look at the outward condition of the MOVs inside, and maybe put a meter across them to see if they're leaking.


There is no outward discoloration, cracking, etc., but I was prepared to cut them out of the circuit anyway.  There are three.


There is also another device - Carli MPXG MF/40
http://www.sjohm.co.kr/products/img/pdf06.pdf
A metalized polypropelene film capacitor in the circuit -  CLASS”X” Capacitors intended for applications where a short will not cause a dangerous electric
shock. (a dead short would pop the included circuit breaker)


I know this is not electronics class, but if anyone is feeling particularly generous, could you indulge my three questions?


1. I'm not sure I trust MOVs, so they may come out of the circuit regardless.
2. The cap is across hot/neutral leads, I'm assuming as some type of noise suppression device. yes?
3. If I were to cut that out of the circuit, what might be the adverse effects or benefits of doing so? 


Thanks in advance,
frank





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Scott Holtzman

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Re: We LOVE MOVs, but what about...
« Reply #1 on: October 21, 2014, 01:27:36 pm »

Hi,
I've read just about every post about MOVs and the risk/reward of having them in a powerstrip. 


I have my rack mounted switched "power strip" open to look at the outward condition of the MOVs inside, and maybe put a meter across them to see if they're leaking.


There is no outward discoloration, cracking, etc., but I was prepared to cut them out of the circuit anyway.  There are three.


There is also another device - Carli MPXG MF/40
http://www.sjohm.co.kr/products/img/pdf06.pdf
A metalized polypropelene film capacitor in the circuit -  CLASS”X” Capacitors intended for applications where a short will not cause a dangerous electric
shock. (a dead short would pop the included circuit breaker)


I know this is not electronics class, but if anyone is feeling particularly generous, could you indulge my three questions?


1. I'm not sure I trust MOVs, so they may come out of the circuit regardless.
2. The cap is across hot/neutral leads, I'm assuming as some type of noise suppression device. yes?
3. If I were to cut that out of the circuit, what might be the adverse effects or benefits of doing so? 


Thanks in advance,
frank

In my opinion the only protection fast enough is silicon avalanche diodes.

I am going to follow your link and take a look at the application data on those caps.

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John Roberts {JR}

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Re: We LOVE MOVs, but what about...
« Reply #2 on: October 21, 2014, 01:41:14 pm »

Hi,




I know this is not electronics class, but if anyone is feeling particularly generous, could you indulge my three questions?


1. I'm not sure I trust MOVs, so they may come out of the circuit regardless.
You need to phrase that as a question.
Quote
2. The cap is across hot/neutral leads, I'm assuming as some type of noise suppression device. yes?
Yes, and perhaps reducing switch noise if the strip has a switch.
Quote
3. If I were to cut that out of the circuit, what might be the adverse effects or benefits of doing so? 
I would not assume that any parts are not useful or needed, otherwise the manufacturer wouldn't pay the money to buy them and install them.

JR
Quote

Thanks in advance,
frank
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Jerome Malsack

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Re: We LOVE MOVs, but what about...
« Reply #3 on: October 21, 2014, 02:42:32 pm »

And the warning label said, "no user serviceable devices inside".  Cutting them out makes this a wire to outlet, extension cord. 
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Kevin Graf

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Re: We LOVE MOVs, but what about...
« Reply #4 on: October 21, 2014, 02:45:08 pm »

The MOV's do one job and the cap's do another.
The best place for MOV's is at the building's service entrance.
At the point of use, having MOV's or cap's to the Safety Ground is dumb.
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frank kayser

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Re: We LOVE MOVs, but what about...
« Reply #5 on: October 21, 2014, 09:36:58 pm »

You need to phrase that as a question. Yes, and perhaps reducing switch noise if the strip has a switch. I would not assume that any parts are not useful or needed, otherwise the manufacturer wouldn't pay the money to buy them and install them.

JR


The strip does indeed have a switch.  Thanks for the info.  I have also seen switched power strips without caps. 
I'm guessing it is somewhat analogous to the capacitor in an old points style automotive distributor.   
As far as certain parts being useful (or not), that's why I asked what its function may be.
I'm also guessing the cap mentioned if leaking, could be a source of noise on the line.


Yeah, the statement about the MOVs was hardly a question.


Discussions in the past about surge protectors, has run not so much in favor of MOVs in surge suppressers.  Folks on this board have stated many times that MOVs are "consumable" devices, and can fail in less than graceful ways - and can be fire hazards when they do, or partially fail leaking voltage to the EGC.  Those same folks also recommend not to have surge suppressors in line one after the other as any leakage to the EGC the strips may have, that current is additive, and can present either safety or noise issues.  No, I do  not plan to have multiple surge suppressors in line.


I've thrown out a number of old "surge suppressors".   Threw them out because they were too old.  Yes, they were still working "just fine"  Post mortem on them showed discoloration around the MOVs, and one with melted plastic.  Some had the regular bar crud that was spilled into them over the years.  They were replaced with power strips, specifically NOT surge suppressors.


As this is a rack "surge suppressor", it is pretty well protected from spills and bar crud.  The service life of these could be increased safely by removing the consumables from within.  Surge suppression at this point in the power distribution is not something I desire.  I need a rack mounted power strip.


So the good and bad of MOVs are what they are - I do not like having what I consider time bombs in a surge suppressor - I will forego the surge suppression, and not have that worry in the back of my mind.


The MOV's do one job and the cap's do another.
The best place for MOV's is at the building's service entrance.
At the point of use, having MOV's or cap's to the Safety Ground is dumb.
My point exactly in clipping the MOVs out of the circuit.  As far as the cap... Jury is out.  If the cap proves more beneficial than a liability, it stays.  Am I second guessing the designers?  Well, I guess so...


And the warning label said, "no user serviceable devices inside".  Cutting them out makes this a wire to outlet, extension cord. 


Precisely.  After what I've read, MOVs are not something I want.  An "extension cord" - a power strip without suppression is what I want.
And, come on.  No User serviceable parts inside".  Indeed.  I guess the amps and other electronics we open up to remove dust and grime also have the same "No user serviceable parts inside" label.

In my opinion the only protection fast enough is silicon avalanche diodes.

I am going to follow your link and take a look at the application data on those caps.

 
Thanks, Scott.


Yes, I asked for it.  And I do appreciate folks taking the time for your responses.
frank
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Mark Cadwallader

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Re: We LOVE MOVs, but what about...
« Reply #6 on: October 21, 2014, 10:02:05 pm »

Based on comments from folks like those in this forum, I purchased a non-surge protected, "non-conditioned" switchable rack mount power strip for my little rack.  Reasonably cheap, wall wart spaces, and enough nema 5-15Rs on both sides of the device to cover all the FOH stuff I have or might need.  Thanks for the collective wisdom, folks. Mark C.
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Jonathan Johnson

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Re: We LOVE MOVs, but what about...
« Reply #7 on: October 22, 2014, 01:22:55 am »

The best place for MOV's is at the building's service entrance.
At the point of use, having MOV's or cap's to the Safety Ground is dumb.

While having some form of overvoltage protection at the service entrance is a good idea, the switching of other loads within the premises can induce overvoltage transients within the building or electrical distribution circuitry. Having overvoltage protection that shunts excess energy to ground at the point of use may still be a good idea. Whether MOVs are suitable for point-of-use overvoltage protection is a different debate.
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Tom Bourke

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Re: We LOVE MOVs, but what about...
« Reply #8 on: October 22, 2014, 04:37:57 am »

A cap line to neutral is going to provide a low resistance path for high frequency noise on the lines. It does nothing for spikes.

MOVS deal with spikes.  There are MUCH better ways to help reduce spikes than MOVS, the problem is they cost at least 10X as much.
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Scott Holtzman

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Re: We LOVE MOVs, but what about...
« Reply #9 on: October 22, 2014, 10:05:14 am »

A cap line to neutral is going to provide a low resistance path for high frequency noise on the lines. It does nothing for spikes.

MOVS deal with spikes.  There are MUCH better ways to help reduce spikes than MOVS, the problem is they cost at least 10X as much.

Spikes can be introduced by motors and such after the mains.  Downstream surge and noise suppression is not a bad idea.  Also lines can induct RF from nearby transmitters, has anyone lived near a high power AM station? 

ANSI/IEEE C62.41-1991 standards define three categories of surge level, based on strategic location within a facilities wiring network, where power problem may be encountered. They classify the surge protector type, the potential impact of transient surge or spikes, and location as follows:

Category A: Defined as any outlets and long branch circuits extending more than 10 meters (30 ft.) from category B location or 20 meters (60 ft.) from category C. Surge protector for this location category is applied at the outlets or individual circuit level for individual protection of a specific piece of equipment such as computers, weighting bridges, measuring equipments, process control equipments and DC power supplies, etc.
Category B: Defined as all major sub-feeders, bus systems, and short branch circuits such as distribution panels, industrial busses and feeder systems, heavy appliance circuits, lighting systems in large building. The protection at this location is very effective in suppressing the much more frequent internally generated transients, ever-changing transient conditions, especially, sensitive equipments and equipments which are fed from the substations.
Category C: Defined as outside and main service entrance which includes main supply lines, transformer, service connections, and feeder line to main service entrance panels, any overhead or sub-feeders lines, underground lines to well pump. This surge protector type is applied to protect against externally caused power disruptions. This installation will help guard against lightning strike entering a facility via the power line.

With regard to device selection:

MOV (Metal Oxide Varistor) composes of zinc oxide material, which is semiconductor with a variable resistance. In normal condition, MOV presents itself as a high impedance device but when voltage is too high, the resistance of MOV drops rapidly to provide a low impedance path of flow. MOVs have finite life expectancy and degrade when exposed to a few large transients, or many smaller transients. MOV is the most common component in AC surge protector.
Gas Discharge Tube (GDT) can divert the excess current from line to ground by using inert gas as conductor from hot line to ground line. In normal condition, the inert gas acts as poor conductor, but when voltage is above the acceptable level, the inert gas is ionized to be effective conductor to pass on current to ground until voltage returns to normal level. GDT will conduct at a voltage less than the high voltage that ionized the gas and able to conduct more current for their size than other components. GDT has a finite life expectancy, and can take a few very large transients or a greater number of smaller transients.
Silicon Avalanche Diode (SAD) provides the perfect limiting action of protective component, but has a lower current capability. When voltage increases above the limit level, SAD will tolerate avalanche breakdown resulting voltage is conducted to ground.
Other important components, such as resistors, capacitors and/or inductors, are used in conjunction with these protector components above.
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Scott AKA "Skyking" Holtzman

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ProSoundWeb Community

Re: We LOVE MOVs, but what about...
« Reply #9 on: October 22, 2014, 10:05:14 am »


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