Checkbox question

[ban-all-sheds]

OK - gonna build my own.

Up to 6 each low continuity and high IR positions available - what values should I pick?

 

[sparkyspike]

I think it's 0.5, 1.0 and 10 Ohm for low resistance and 0.5, 1.0 and 10 MegaOhm for IR

 

[RF Lighting]

Do you want to build two?

 

[Cremeegg]

BAS - get one off Fleabay for the price of a small round of drinks - far easier than making your own - more time to spend down the pub.

 

[ban-all-sheds]

Do you want to build two?

 

[ban-all-sheds]

BAS - get one off Fleabay for the price of a small round of drinks - far easier than making your own - more time to spend down the pub.

Ah, but this isn't about price, or ease - I shall enjoy making it.

 

[NotHimAgain]

A lot depends on what you are trying to achieve with the check box. If it is simply to provide an indication that an instrument is 'probably' functioning as well as it did when you got it - then a few simple test resistors will be fine. If you expect more (and I doubt that you do) then it can all get much more complicated.

Assuming a simple check box - a good starting place would be the ranges over which it is supposed to work - in my case this is also influenced by what value resistors I happen to have hidden in the garage!

For insulation resistance I often use:
0.25 M Ohm
0.50 M Ohm
1.00 M Ohm
2.50 M Ohm (because I have a resistor of that value )
10.00 M Ohm

I don't bother going higher because I have serious doubts about achievable accuracy (due to testing method errors) once the readings get much higher than this.

My test resistors happen to be high stability 1% types. Note: My check box is actually a few of these with one end twisted together and taped with stick backed plastic - some say I don't take this subject seriously - but what do they know

For continuity I use some very accurate (0.05%) non inductively wound, high current resistors (over 1 A). These have various values below 1 ohm. I just happen to have these and I am not suggesting that they are essential. I would probably look for high stability wire wound 1% resistors with as high a current rating as possible within price constraints.

The sizes would be in the range of the instrument so around 0.01 to 5 ohms for practical testing.

I don't know what you intend for RCD and Loop Impedance - both are difficult.

RCD checks tend to involve a calibrated test current and a "time measurement" (actually a measurement of output voltage reduction).

Loop impedance testers are a joke - but most test boxes check them by adding resistance (funny for an instrument that is supposed to measure impedance). I do have a 'proper' test box and I think that adds 5 ohms (it is a long time since I bother using it )

 

[Grizzly]

Here's a guide to a 'DIY' checkbox from another forum:
http://www.talk.electricianforum.co.uk/showthread.php?t=12

 

[lazygit]

Here's a guide to a 'DIY' checkbox from another forum:
http://www.talk.electricianforum.co.uk/showthread.php?t=12[/QUOTE]

The instructions you refer to are similar to a box available on ebay for 20 quid, yet the make your own version costs over a tenner in parts alone

Surely there are better things to do with your free time?

 

[securespark]

I once compiled a website, having learnt every letter of HTML code from scratch.

I did this because I had the time & the inclination to learn about the workings of a website and the language.

I fully realise I could have achieved the same (nay better) results using other methods, but that was what I wanted to do.

 

[jockspark]

hi have a look on ebay there is a calibration card the size of a credit card
i know you want to build your own the there is a good picture of it the values and the set up may be helpful to your project plus tell me your thoughts on it i am thinking of buying one not hijacking your thread but some feed back would be helpful

 

[ebee]

might I suggest a little thought such as.
What values are most the useful? ie

1/ lowest I might reasonably encounter
2/ the highest value I might reasonably encounter
3/ a reasonsable spread between 1/ & 2/
4/ the highest value I need to give an indication its in the vicinity

say take ins res under the 16th (17th is a bit different).

0.5 M or less is a no no.
2.0 M is OK but might require an investigation.
1.0 M is somewhere between

100M or 200 M 0r 500M or 999M is what my meter might read as maximum

Also Personally I don`t trust owt under 10M on an existing installation anyway

So 0.5m, 1.0M, 2.0M, 10M & say 500M might be some gud uns to start with and then say pick the nearest from the preffered value range, high stabillity but not nessacarily close tolerance.

Then say R1 + R2 of a 88 metres of ring final might give R1 of about 0.65, R2 of about 1.06.
50 metre ring say 0.37 & 0.605
46 metres of 1.0 say 0.83
46 metres of 15 say 0.56
20 metres say 0.36 & 0.24

so 0.24, 0.37, 0.6, 0.83 & 1.0 might be sum gud uns and once again nearest preffered.

Hope that helps a bit,

Yes I built me own too plus EFLI (cheated a bit) & RCDs to boot, it impressed my assessor, especially when I said "It might suprise you to learn I did not buy this I made it meeself!" - It looked just like I`d built it and definately not bought it - he saw my humour LOL

 

[ban-all-sheds]

A lot depends on what you are trying to achieve with the check box. If it is simply to provide an indication that an instrument is 'probably' functioning as well as it did when you got it - then a few simple test resistors will be fine. If you expect more (and I doubt that you do) then it can all get much more complicated.

Why do you doubt that?

Actually all I do want is to verify consistency - not traceable back to national standards, but more than just a few simple test resistors...

For insulation resistance I often use:
0.25 M Ohm
0.50 M Ohm
1.00 M Ohm
2.50 M Ohm (because I have a resistor of that value )
10.00 M Ohm

I don't bother going higher because I have serious doubts about achievable accuracy (due to testing method errors) once the readings get much higher than this.

Similar to my initial thoughts, although I was also going to go higher - accurate or not one does see higher values IRL..

My test resistors happen to be high stability 1% types. Note: My check box is actually a few of these with one end twisted together and taped with stick backed plastic - some say I don't take this subject seriously - but what do they know

It may work, but it's a bit tacky....

For continuity I use some very accurate (0.05%) non inductively wound, high current resistors (over 1 A). These have various values below 1 ohm. I just happen to have these and I am not suggesting that they are essential. I would probably look for high stability wire wound 1% resistors with as high a current rating as possible within price constraints.

My tester delivers a few '00 mA at 5-9V, so I was going to use resistors of a few watts..

I don't know what you intend for RCD and Loop Impedance - both are difficult.

RCD checks tend to involve a calibrated test current and a "time measurement" (actually a measurement of output voltage reduction).

Well I'm certainly not planning on building an adjustable RCD simulator that the posh proprietary boxes have - I was just going to have an actual RCD in there.

Loop impedance testers are a joke - but most test boxes check them by adding resistance (funny for an instrument that is supposed to measure impedance).

Funny or not that's what they do, so as long as I can show that it does the same funny thing each time...

I do have a 'proper' test box and I think that adds 5 ohms (it is a long time since I bother using it )

I'd been thinking 1 ohm, rated at 200W or so....

 

[Grizzly]

The instructions you refer to are similar to a box available on ebay for 20 quid, yet the make your own version costs over a tenner in parts alone

Surely there are better things to do with your free time?

Well, if I were wanting a checkbox, I would certainly want to make it myself, rather than buy it off ebay, regardless of any costs or savings.

 

[NotHimAgain]

It sounds like you have a reasonable plan .

On higher value insulation resistance values (above 10 M Ohm) - first I would recommend getting a copies of some of the publications on the Megger site (http://www.megger.com/uk/publications/index.php)- you will have to register and login.

Read StichinTime.pdf for starters.

This will inform you about some of the problems that are inherent in 'real world' insulation resistance measurements. Why do I say 'real world', well when you have an instrument calibrated or use a check box you are testing against a physically small resistor, possibly in a metal box. Contrast this to testing a group of circuits connected to a distribution board (I trust you are familiar with 612.3.2).

Picture the 'resistor', it consists of long lengths of cable orientated in many directions. These should act as a pretty good receiving antenna . Then there are all the joints that may introduce thermal currents etc, etc. Now divide 500 by 10,000,000 and tell me what the answer is - this is your test current (big init !). Add this to the information in StichinTime.

Now tell me again how you are going to use your tester to measure 'real world' insulation resistance values in excess of 10 M Ohms with any reasonable degree of certainty

Now this does not negate your test box - if it is properly designed and constructed it may well be able to give a reasonable check of you instrument for values over 10 M Ohm - but my point is - is it worth it?

Loop and RCD testers are another issue and what you have planned is similar to what you get in most test boxes on sale - I have a number of reservations about the check boxes and the instruments.

However, my hang ups are not really going to help you so maybe we should leave these for another day.