Transformer Impedance Computations

[gar]

1812191952 EST

tersh:

You have a tendency to take statements out of context, and try to apply them under some unrelated conditions.

You have to get down to basics, and understand how to apply those basics to your problem at hand. In others words accurately define your circuit, and then figure out how to ask the correct questions relative to that circuit.

You probably don't want to define an RLC circuit on earth, and then from this try to determine the temperature on the moon. These two don't have much to do with each other.

.

 

[Russs57]

If you had a source impedance of ten ohms on that 20 amp 240 service....and you put a 15 amp 240 load on it.....the load would running at 90 volts......so I rather doubt you have a 10 ohm source impedance.

Why worry about an 8/20 event a mile away? The event won’t travel that far.

 

[winnie]

Winnie, about this "

source of 'surge' is induced voltage caused by magnetic fields from nearby events.". Were you referring to the typical surge caused by lightning, for example the 6000v, 3000A, 8/20 usec waveform surge? [...]
Btw.. I'm familiar with MOV specs like MCOV, VPR, etc. so didn't give the line voltage because I just want to know specifically what impedance between MOV and load can do. Thanks.​

I said that the impedance 'downstream' of the MOV is not significant for line-line surges coming in on the power lines.

I said that the impedance 'downstream' of the MOV is important if the surge is somehow coming from the downstream side. This would not be a typical lightning surge, which is imposed on the power lines when lightning hits the distribution system, but rather the much rarer situation of induction from a very nearby lighting strike.

Impedance 'downstream' is also important if you have multiple lines converging on the equipment you wish to protect, eg. power and phone lines leading to a computer. In which case you are concerned not only with the power supply voltage and the phone line voltage, but also the voltage _between_ power and phone.

-Jon​

 

[tersh]

1812191952 EST

tersh:

You have a tendency to take statements out of context, and try to apply them under some unrelated conditions.

You have to get down to basics, and understand how to apply those basics to your problem at hand. In others words accurately define your circuit, and then figure out how to ask the correct questions relative to that circuit.

You probably don't want to define an RLC circuit on earth, and then from this try to determine the temperature on the moon. These two don't have much to do with each other.

.

UL has a requirement that there must be at least 10 meters between type 2 and type 3 SPD. Many theorize the purpose is so the type 3 SPD would have good source impedance (the 10 meter wires). But couldn't the type 3 SPD also feel the source impedance (utility transformers, etc.) of the type 2 SPD too? If yes, why is the 10 meters minimum required?

 

[gar]

181219-2507 EST

The following references are not what I wanted to find, but are interesting.

https://en.wikipedia.org/wiki/Spark_gap
http://ethesis.nitrkl.ac.in/2875/1/Full_Thesis_Print_04.07.2011.pdf
http://edisontechcenter.org/LightningSuppression.html

.

 

[gar]

181220-0734 EST

tersh:

I don't believe you have yet developed a basic intuitive understanding of how a transient limiter works. If you did, then I believe you should understand the 10 meter requirement. This rule probably implies that 10 meters of branch wiring provides more high frequency impedance than is seen at a main panel. More impedance means less peak current in a voltage source circuit.

You need to know that that UL, NEC, and other organizations are rule based systems. You do things based on a set of written rules. Similar to law books. These rules are based on theory, empirical data, and probabilities of being useful.

You may or may not have read the following, and it may not help your understanding:
https://www.nemasurge.org/wp-conten...cation-Considerations-Rev-Date-01-31-2013.pdf
Note that the 10 meter rule comes into play on limiters with lower peak current capability. Really this is an energy dissipation capability.

.

 

[tersh]

181220-0734 EST

tersh:

I don't believe you have yet developed a basic intuitive understanding of how a transient limiter works. If you did, then I believe you should understand the 10 meter requirement. This rule probably implies that 10 meters of branch wiring provides more high frequency impedance than is seen at a main panel. More impedance means less peak current in a voltage source circuit.

You need to know that that UL, NEC, and other organizations are rule based systems. You do things based on a set of written rules. Similar to law books. These rules are based on theory, empirical data, and probabilities of being useful.

You may or may not have read the following, and it may not help your understanding:
https://www.nemasurge.org/wp-conten...cation-Considerations-Rev-Date-01-31-2013.pdf
Note that the 10 meter rule comes into play on limiters with lower peak current capability. Really this is an energy dissipation capability.

.

Thanks for your help. Tomorrow I'll go to vacation in the countryside for a week with my parents and in between picnics in the mountains, I'll reflect on everything you said and review lessons 25 years ago. In my country, all SPD installers (including licensed electrical engineers) consider MOV as just something to dump or throw the current to ground.. they never understood the relationship to impedance because in our college we were never taught about MOV so being non bright without any Manhattan Project level infused surrounding and ambience. We were not used to any original or creative thoughts.

SPDs are actually rare in homes or normal office. It's only installed in facilities like telecommunications site, servers and stuff. And if you will check them. You will commonly see 320v MCOV being installed from line to ground (120v) which should need 150v MCOV. They would tell you MCOV is the VPR or let through voltage. So even in the specs, they were confused already. One of them tried to convinced me it was so. This was when I sought help from a 65 year EE in the US on how to use the right product. This was how I was able to get the Siemens First Surge 140,000A. The best type 2 SPD in the USA for home use. Now you increased my understand of SPD twice over.

Thanks to you and others. Have a Blessed Christmas!

Cheers.

 

[mivey]

Fault current? We were describing surge current.. not GFCI ground fault current.

I was just asking what would happen to the MOV (shown in green) when it's between the source having great impedance and say a transformer near the load with great impedance. The value is just for sake of computations and not signifying anything. Assume a surge of typical 6000v, 3000A, typical surge waveform hits it. It's just an example of the surge current suppressed at the MOV.

Given your circuit, valid or not, most of the voltage is dropped across Z1.

You have a low impedance (the 1 ohm shunt) in parallel with 20 ohms so most of the current flows through the red source impedance and green shunt and just a small amount (current divider circuit) through the load.