Ufer grounds

[roger]

Re: Ufer grounds

Dereck and Karl, with my limited knowledge of lightning and I think this can be said of even the experts, who knows?

I have always thought of lightning being rapid rise DC which could mimic AC in the single rise event. If that makes sense.

Sorry, I know I'm out of my league here, but I can't see lightning (a big static charge) as being AC.

Roger

 

[dereckbc]

Re: Ufer grounds

Roger, any arc be it from lightning, high voltage AC or DC, static, etc contains elements from DC to several hundred megahertz. So it is a mixture. You are correct about there being DC components, but the fast current rise time mimics AC say 10KA in 5 microseconds equates to 200 KHz.

Karl, most surge events are in the differential mode as opposed to common mode. The events occur ahead of the transformer, so when passed through the transformer the potential is from phase-to-neutral, rather than phase-to-ground.

This is why us so called experts advise that your service entrance TVSS only needs protected L-N, the other modes are redundant and only add cost. The common modes like L-G and N-G are already covered by L-N because of the N-G bond requirement on single phase and 4-wire 3-phase services.

Common mode disturbances are generated internally or by a direct strike via lightning hitting the structure. So with this in mind that it why it is advised to use”all-mode” (L-L, L-N, L-G, N-G) devices on sub-panels and end equipment. Keep this in mind, that if it is lightning, the impedance of the EGC's back to the entrance GEC is long and exhibit a high impedance. This is why you see some industries like mine (telecom) use ground grids and bond every sub-panel ground bus directly to the grid in addition to the NEC EGC requirements. This additional bond is much shorter and has much lower high frequency impedance to earth than the EGC circuits.

Hope that helps. Dereck

[ August 21, 2003, 08:09 PM: Message edited by: dereckbc ]

 

[roger]

Re: Ufer grounds

Thanks Dereck. If you ever wake up and think you need to change professions; instructor, teacher,
or professor would come natural to you.

Roger

 

[dereckbc]

Re: Ufer grounds

Roger, thank you. I have heard that from a few other members. I was recently unemployed and tried to break in to teaching, but most want MSEE with teacher accreditations/certifications. All I have is BSEE and 25 years experience. Now for the big laugh, I was hired as a contractor for the company I was laid-off from doing the same job for more money.

Seriously though I would love to teach or work for an A/E firm.

 

[karl riley]

Re: Ufer grounds

Dereck, excuse me if I am stretching this thread. Is it NEC compliant to put a "blocker" between a CATV sheath and the building grounding point? A CATV company said that is their solution to keeping the building's neutral current from using their sheath to go to the next building and their neutral, but the blocker will close if high voltage wants to get through to ground.

Karl

 

[dereckbc]

Re: Ufer grounds

Karl, I assume you are talking about some sort of isolation transformer used to block neutral current from traveling along the sheath. I do not see any reason for a code concern as long as the output side of the sheath is bonded to the GEC.

I am not in the CATV buisness but again I assume they use simular devices as a telco uses to isolate ground systems.

The device on my home is an isolation transformer with built-in SAD arrestor. The device is bonded to the GEC. I have taken it loose and measured for continuity between input and ouput, there is no continuity from input center pin to output center pin, or input ground to output ground.

[ August 21, 2003, 09:52 PM: Message edited by: dereckbc ]

 

[pierre]

Re: Ufer grounds

Dereck
Since this thread has taken on a new twist into TVSS, I have questions also. So far this has been very informative.
Lets say we have an office with copiers, printers and A/C units which cause disturbtances within the premise wiring (load side of the transformer).
For this type of disturbance, is the grounding electrode and grounding electrode conductor resistance important? Would this be a common mode surge as opposed to differential?
What is the difference of common mode/differential mode?
Thanks

Pierre

 

[brian john]

Re: Ufer grounds

Pierre:

Couple of books for your library:

Mark Waller's:
Surges Sags and Spikes

Allan Greenwood's
Electrical Transients in Power Systems
(second edition)

If you only buy one book start with Waller's book.

 

[dereckbc]

Re: Ufer grounds

Pierre.

Common mode is the voltage that appears equally and in phase from each current-carrying conductor to ground.

Diffential mode (a.k.a transverse mode, or normal mode) is voltage between or among active circuit conductors feeding the subject load, but not between the EGC or associated signal reference structure and the active circuit conductors.

 

[karl riley]

Re: Ufer grounds

Pierre, what is the effect of the disturbances that you mention? If the circuits supplying the office equipment are misconnected so that they are producing net current fields, that is sometimes the source of disturbance. How did you measure the disturbance?

Karl

 

[pierre]

Re: Ufer grounds

Brian
Thanks, I ordered Mark Waller's book and it will be here monday.

Dereck
Thank you very much. The explanation was very helpful and informative.

Karl
I was refering to harmonic disturbances from nonlinear equipment.

Before computers and the internet were available to us, I have no idea how I would have been able to get such good info so easily. Thanks to all who put up with my %^*$$^ and provide me with such great 'stuff'.

Pierre

 

[iwire]

Re: Ufer grounds

This has been really interesting following this thread, a lot of great info. thank you to all involved.

Dereck, glad to hear of your change of fortune.

Originally posted by pierre:
Lets say we have an office with copiers, printers and A/C units which cause disturbtances within the premise wiring (load side of the transformer).
For this type of disturbance, is the grounding electrode and grounding electrode conductor resistance important?

I am curious about this too, it is my feeling that the grounding electrode system will have no impact on disturbances caused by the line to line loads.

Any disturbances caused by line to line loads have to go back, dissipated / shunted / directed, to the source, not to the earth.

Is this a correct assumption or does the earth play a roll?

Thanks, Bob

[ August 22, 2003, 03:44 PM: Message edited by: iwire ]

 

[dereckbc]

Re: Ufer grounds

Bob and Pierre, thanks. The end equipment generates the harmonics you speak of, as you probable know. The switching DC power supplies used to supply the electronic circuits causes it. These DC supplies are highly non-linear. Can EGC and GEC impedances affect the magnitude? No!

Manufactures can add filtering to minimize the problem, but due to the competitive nature of the business, only minimal efforts are taken if even at all. The biggest problem I have witnessed is overheating of transformer cores, and in some cases excessive current in the neutral conductors of poly phase systems where the neutral was not sized properly for non-linear loads.

As far as the affects on grounding, some manufacturers install filters from L-G, and N-G to comply with FCC regulations. Filters installed this way rather than L-L and L-N do cause currents to flow on the EGC?s back to the source transformer. This will cause some ?common mode? noise problems. The common mode noise can affect some equipment that uses I/O signal cables that use ground as a signal reference in particularly RS-232 and A/V cables to name a few.

WHAT THIS HAS TO DO WITH UFER GROUNDS I DO NOT KNOW?

[ August 22, 2003, 05:18 PM: Message edited by: dereckbc ]

 

[karl riley]

Re: Ufer grounds

I hope Mr Uffer will excuse us. This is very informative. Dereck has the good explanations of what can transpire in the conductors.

Pierre, I am pressing you, since I am interested. I still didn't hear what the disturbance is and how it was measured. I only re-ask this because I have found it is necessary to distinguish between noise in the conductors (harmonics, etc) and the induction noise in the magnetic field coming from net currents. Net currents are not caused by harmonics, but if there are triplens harmonics (180, etc) there will be more neutral current, so if you do have some shunted neutral current the magnitude of it and its field will be larger than with linear loads.

Karl