High Impedance Protected Test Points for 115 VAC

[4x4dually]

Getting into something that I've not done before and thought I'd ask if anyone has delt with this type of design. Any constructive feedback would be greatly appreciated.

I'm designing a test panel that will allow DMM probing on the 115 VAC circuits. This probing will allow personnel the ability to fault isolate bad components without the need to remove equipment. The circuits will be 115 VAC line to return; however, when installed on the ships, it will actually be 58 L-G, 58 Return-G. Since there isn't a bonded "neutral", I think I need to current limit the line and return points equally.

I would like to provide user safety by conducting voltage readings or absence of voltage testing without the risk of accidental contact with live electrical components. I'm assuming this could be accomplished with something as simple as an in-line resistor but I'm just now starting to research how it is done. I always over simply things so I thought I'd ask if anyone has a good working knowledge of this setup.

I've been digesting this page but as with most pages, they talk a lot of jargon and never give an approved circuit or method for actually doing it. Typical of most regulatory bodies....they like to make rules but they can't tell you how to meet them.

Understanding ‘Point of Work’ in NFPA 70E -- Occupational Health & Safety

How permanent absence of voltage test points ensure compliance and safety.

ohsonline.com

Any feedback would be great!!!

 

[winnie]

Are you talking about devices such as this:

https://www.standardelectricsupply.com/blog/safe-test-point

 

[4x4dually]

Exactly devices like those....but I need to know what is in those that qualifies them as "protection devices" so that I can implement the same circuitry inside our design. Since there isn't any real design info except the visual of the box here, I think I'm safe sharing my concept image with you. It's nothing more than a box with a PWB full of LEDs and test points. The cable basically extends the traces from our main PWB assembly that isn't accessible to the user without tearing the equipment down. The LOTO on the ship can take 10 minutes to 4 hours and is a royal PITA. It would be nice to probe with power connected and remove the LOTO issues. The LEDs and Testpoints are already in use in this equipment is the reason I picked them. They are already approved. It would be sweet to add indicators to the AC as well, but I'll have to figure that out as well.

 

[winnie]

Ok, this is well outside of my training, so I hope someone else can chime in.

The datasheet for the device I linked gives the internal resistance values and a UL file number.

I'd want to get the UL standard used for these test points.

My gueses: 1 the resistance is selected so that a worst case shock through the test point is 'safe' (the units linked had a resistance of 102K for a maximum voltage of 600V, giving about 6mA into a short, compare that to GFCI requirements). 2 the resistors have a high enough power rating to survive a short on the output. 3 Fuisable resistors are used to ensure they fail open. 4 there will be barrier and creep distance requirements between different test channels.

Hope this helps.
Jonathan

 

[4x4dually]

All go info and design points. Thanks for chiming in. We will have to do some research for sure.

 

[Besoeker3]

For UK and EU (around 300 MW) we use just 230V for domestic supplies.

 

[4x4dually]

For UK and EU (around 300 MW) we use just 230V for domestic supplies.

I'm confused as to how that relates to the issue.

 

[Besoeker3]

I'm confused as to how that relates to the issue.

It is simple compared USA complexity.

 

[winnie]

This isn't USA complexity. This is for a power system on a ship. +-58V single phase, 115V L-L.

The UK might be simple for building wiring (230V single phase, 400V L-L three phase, nothing else commonly used in buildings), but you will certainly find a mix of other voltages used in niche applications there. 110V is apparently a standard for 'trade tools' (hand held portable tools, fed by a transformer for safety), and the aircraft in the UK use the same 200/110V 3 phase 400 Hz that they use the world over.

Back to the OP: running LED indicators from the AC voltage is pretty simple if you don't mind a bit of flicker. I think you can even get LED modules which have the rectifier and voltage drop resistor built in. You might have trouble getting standard parts rated for 58V, here is an example of a 120V unit: https://www.digikey.com/en/products/detail/bulgin/LE2951WL5G/2747949

-Jonathan

 

[4x4dually]

So, breaking it down to an ohms law calculation....the top diagram would be if we were to short the test point dead across. R1 would have to be capable of more than .12 Watts and the resistance of 120K would limit the current to 1mA. That would regulate it to around the human body threshold of sensation. Add the resistance of the human body to it on a good wet, heavy sweat day, with no shoes on, and I still doubt you'd even feel it as any body resistance would only limit the current even more. That should regulate the current to less than most GCFI trip settings of 4-6mA.

Diagram 2 would be adding a DMM with 10Mohm impedance. Current would drop to around 10uA and we'd lose 1.5V of reading across R1.

Diagram 3 would be if we split the R1 value on both sides of the test points since we are dealing with split-phase (of whatever slang you wish to call it) 120 L-L, 60+60, yada yada yada.

It seems simple yet, I'm sure there's more to it than meets the eye right out of the gate. Just trying to get my ducks in a row in case our proposal is accepted, and we have to go into full on design.

 

[Besoeker3]

This isn't USA complexity. This is for a power system on a ship. +-58V single phase, 115V L-L.

The UK might be simple for building wiring (230V single phase, 400V L-L three phase, nothing else commonly used in buildings), but you will certainly find a mix of other voltages used in niche applications there. 110V is apparently a standard for 'trade tools' (hand held portable tools, fed by a transformer for safety), and the aircraft in the UK use the same 200/110V 3 phase 400 Hz that they use the world over.

-Jonathan

Most of the systems I used were industrial - paper mills, industrial water systems, typically for 11kW.
For domestic it is all 230V

 

[4x4dually]

And I thought I was the king of the thread derail. LMAO I've been de-throned.

 

[winnie]

It seems simple yet, I'm sure there's more to it than meets the eye right out of the gate. Just trying to get my ducks in a row in case our proposal is accepted, and we have to go into full on design.

I agree with this. The basic features are simple in the extreme: you have a 'test point' which you can access easily, protected by a resistor to limit current to a non-hazardous level. I'm sure the devil is in the details which are not obvious to someone not trained in the particular requirements of the task (meaning me).

Good luck!

 

[4x4dually]

Thanks. Just presented the proposed design to the customer and the big boss liked it so looks like we will move forward with the plan once funds are made available. In the mean time, if anyone has any words of wisdom, toss 'em out!

 

[Frank DuVal]

Fluke has an input impedance of what, 11 megohms? If you put a 60K ohm or 120K ohm resistor in series with every test point the Fluke will not know it is there!

11 megohms + 100K ohms = no appreciable voltage drop change in readings.

This is an old trick of radio repairmen years ago with the 1K to 20K ohms/volt meters before they could afford one of those king of the hill expensive VTVMs (Vacuum Tube Volt Meters), the trick being to put a 100K ohm resistor at the end of the positive test lead and use it to read voltages in circuits without loading them.

 

[Open Neutral]

Why not a voltage divider of 2 resistors so the TP voltage is 1% of the line?

 

[4x4dually]

Why not a voltage divider of 2 resistors so the TP voltage is 1% of the line?

That's basically what it is. A voltage divider of the limiting resistor and the impedance of the meter. Did a little benchtop test on my desk the other day and 60K on each leg seems to give great results as calculated. They limit current to right at 1mA when dead shorted and to around .001mA when read with a meter.

 

[petersonra]

The article you linked to was written by a manufacturer of such devices. I would keep that in mind when considering what was in the article.

In the USA, you cannot just make these kind of things yourself. They have to be made to a specific UL standard and listed accordingly.

I suspect UL requires more than just a resistor or two. You might want to take a look at the UL standard and see what it requires.

 

[4x4dually]

Luckily, our equipment doesn't require UL listings. It does require Mil-Std-1399 testing and all other mil-qual such as 810, 901, 167, etc.

 

[4x4dually]

In the USA, you cannot just make these kind of things yourself.

To expand on your reply, we CAN just make these things ourselves because we are the OEM of the equipment. We get paid to design, build, test, and service our equipment. Rest assured, we will research and test this setup extensively and I'm sure we will eventually get into what the UL requires even though our customer does not require UL testing. This thread was just asking if anyone else has already done this research in an effort to save me a lot of time which saves the US citizens tax dollars.