Testing the TK Tar Baby

[MileHigh]

Rosemary:

On loan I would NEVER subject the LeCroy to the risk of applying it without ground.

Another ASSUMPTION where you demonstrate your vast ignorance with respect to the field of electronics.  You use that term "ASSUMPTION" as a hot-button word when you try to counter a point that someone is making to you.  However, we know your secret Rosemary.  Your secret is that you are clueless with respect to electronics and therefore you make ASSUMPTIONS all of the time.  You have to make ASSUMPTIONS because you barely know what you are doing.  You need them as part of your coping strategy.  I can't recall a single time when you sincerely asked for advice on how to do something - so ASSUMPTIONS it is.

And sometimes it's comical to see the kooky perplexing situations you get into because of your ignorance translating into an over reliance on ASSUMPTIONS.

The LeCroy will work JUST FINE without a ground plug.  Your ASSUMPTION is wrong.  The ground plug is to prevent risk to you, not risk to the equipment.

Dear lord, there are no words.

MileHigh

[picowatt]

Are there any cases where Lecroy recommends not having the ground connector attached to a ground during use of the equipment? If so, then I would recommend following that procedure. ;]

Mags n The Moonies

Magluvin,

The third wire AC ground connection is there to ensure that in the event of an internal power supply or wiring failure where the isolation between the AC mains and the chassis is compromised, the third wire will return current to the breaker box and blow the mains breaker serving the faulted equipment.  This reduces the risk of having a hot chassis and subsequent electrocution hazard.  It is usually required code that all equip connected to the AC mains have their chassis connected to this safety path back to the mains (in countries that use such a system) or use a code rated double insulation system and a two prong plug (as is also common).  As it is code, all manufacturers must give the same warning to use only with the third prong connected to a separate and low impedance path to the AC neutral at the mains panel on equipment using a third prong.

However, it is often necessary when using test equipment, audio/ video equipmet, etc. that the chassis ground path be broken at the AC cord to eliminate "ground loops".   A separately designed ground path for such installations allows the chassis of all equipment to be held at the AC distrib system ground in the event of a fault, while breaking any loops that may cause issues.  Often though not always wise, signal shield wires are used as the AC fault path, which, if sized appropriately can suffice, though a separate parallel run conductor is usually wiser.  Keep in mind, fault currents are usually brief, being the amount of time required to trip a mains breaker.

Ask anyone who has done audio/video installs to any degree, and they will surely agree regarding the necessity to break loops.  Transformers can sometimes be used in the signal path or in the mains circuit to eliminate a loop, or the shield in the signal wires disconnected at one end.  But, sometimes it is necessary in multi-equip installs, test or otherwise, to have one piece of equip tied to the AC ground and ensure all chassis leakage currents flow to that piece of equipment with separate parallel conductors following  the same path as signal conductors (so induced currents are similar and cancel).  Most test equipment has a chassis ground terminal to which a separate, sufficiently sized wire can be attached for the purpose of connecting the chassis to a system ground point, which may be at the device under test if that is called for.

In Rosemary's case, if she is only going to be using a single 'scope and FG for the new tests, I doubt the need to isolate AC is even necessary for the LeCroy.   I would likely isolate the FG instead.  The problem in the RA circuit is that if the FG signal ground is tied to the non-battery end of the CSR, and the 'scope signal grounds are tied instead to the battery negative, the CSR will be effectively shorted by the AC chassis grounds if the 'scope and FG are not chassis isolated from each other.  This could be eliminated by tying the FG signal ground to the battery negative as well, but then the DC bias current flowing thru Q2 would not pass through the CSR and would flow unobserved.  If safety is an issue, a sufficiently sized wire can be run from the FG chassis parallel to the FG signal cable and also attach to the non-battery end of the CSR, along with the FG signal common from the BNC cable.  Another sucfficiently sized wire can be connected between 'scope ground (or a separate run to the AC ground) and battery negative.  The FG AC ground can then be safely floated.

In the event of an AC fault in the FG, AC fault current will flow through the CSR to the 'scope's chassis ground (or to the separate wire ran to the AC ground from battery negative).  As the .25 ohm resistance will attempt to draw several hundred amps from the AC mains in the event of an internal FG chassis to AC mains short, the mains breaker will trip fairly quick, placing a short high current pulse on the CSR which it would very likely survive.  Its wattage could be increased if there is additional concern, but, honestly, in today's modern equipment, mains to chassis shorts are fairly rare.

PW

[picowatt]

That's because "you" know what you are doing.
If you did have a problem, though, you'd probably have to pay for the instrument, and then to use it after repair you'd need to get it recalibrated.

TK,

The only problem that would be encountered is if a piece of rental equip lost it AC to chassis isolation.  That would be a fault with the rented equiment, and hence not my problem (unless of course it "zapped" me!).

PW

[Magluvin]

TK,

The only problem that would be encountered is if a piece of rental equip lost it AC to chassis isolation.  That would be a fault with the rented equiment, and hence not my problem (unless of course it "zapped" me!).

PW

Beyond what circuit cellar wizards do, Lecroy, a company that designs and makes the equipment, do they recommend ditching the ground termination during any time of use, and if so, what do they have to say about that?
This company must understand these measurement protocols and also understand the necessity for isolation in given cases such as this. These are the people to ask. What they say in the matter should be golden. Otherwise, what could we say about the quality of the equipment?

Mags n The Moonies

[powercat]

Rosemary, I could perform my measurements with a string and a piece of modeling clay and they would STILL be better than yours, because I know how to add and subtract, and how to interpret measurements.

You on the other hand think that a 12 volt battery is fully charged as long as it reads over 12 volts on your multimeter.

And you think that a Watt is a Joule and a Joule is a Watt per Second.

Not only that, my challenge still stands: I'll put my measurements up against yours at any time. Let any third party set out a table of unknown electrical quantities, and let you and i both measure them, one shot, no outside help, starting NOW. How will you perform? How will I?

Anybody want to start a betting pool?

Rosemary odds on being right....... Googolplex to 1.....place your bets The time it would take to write such a number also renders the task implausible: if a person can write two digits per second, it would take around about 1.51×10⁹² years, which is about 1.1×10⁸² times the age of the universe, to write a googolplex.

Betting on TK being right......... is suspended......no more bets