I've decided that I should add a resistance tester (generically known as a Megger) to my toolbox and have been surprised at how few there seem to be available - given that every electrician should(?) have one for testing.
TLC have one for about £80 - can anybody recommend this or another one for occasional use (2 - 3 times a year)!
Peter
Peter Andrews wibbled on Tuesday 17 November 2009 17:07
Or do you want to do a full range of installation tests as opposed to just high voltage resistance tests?
If so, then you'll be looking at a Multi Function Tester, or seperate instruments for:
High voltage resistance tests (you want a 500V test for IEE purposes, some do 250V and 1000V)
Low ohms impedance test - for measuring L-E and L-N impedances of finished dead circuits.
RCD tests - measure the trip time and optionally the trip current of RCDs at various multiples on In (trip rating) and phases (0 or 180 degrees).
Supply impedance test/ PFC test. Optional as you may assume the worst case values but useful.
Earth rod test - only needed if you have an earth rod (obviously) and not often found on mainstream multifunction meters.
If you want a basic high voltage resistance test, there are many meters as you've noticed.
For multifunction, I like the Megger 1552 I have, discount 2nd hand from ebay. That will still set you back a few hundred though...
If you can clarify and say what your intended use is, I'm sure more opinions will be forthcoming.
If you dont need to use it to fill any forms, you can make your own in minutes for a few quid.
NT
I think a lot of sparkies have moved to an all-in-1 tester that does everything from insulation to RCDs to checks the temperature of the tea.
For 'serious' work it needs to have a valid calibration certificate. Instrotech charge from =A327.00 to =A372 depending on whether it's a basic insulation tester or a multifunctional one.
You can hire a basic insulation tester from =A320 + carriage + VAT per week from
Owain
Ebay...
- Robin insulation tester w/continuity test
- Megger rcd tester
Make up...
- Your own wandering lead re R2 figures
You do not need an EFLI (Earth Fault Loop Impedance) tester, you can calculate Zs from Ze (based on supply type limits) + measure R1+R2. Unless you have a big welder or large HVAC unit Zs will not likely be a proble - more important to verify ring/radial continuity and then resistance is within limits re any bad connections.
If you DO buy an EFLI meter - 1) the non-trip type use a 15mA test current which will not test corroded armour or steel conduit continuity adequately (in my view only!) 2) the high current tests use
20-21-25A which will trip an RCD which requires bridging of the RCBO/ RCD.On balance bothering with an EFLI tester is not important enough. Far better to do the IR & Continuity + RCD tests, if you use a lot of power tools & particularly hedge trimmer I would consider an RCD tester pretty essential. Fluke SM300 will at least test RCD operation
- it's just a resistor to cause a 30mA nominal leakage, it does not confirm trip trim like a proper tester.
Individual tools are good in that if something fails the replacement is confined to that tool - repairs can a) be a con re badly done b) be as much as a new machine certainly on Ebay. Multifunction tools are favoured because they can record for download later, direct into forms and so on.
Realise the limits of an IR tester. Air is a perfect insulator - thus the most grotesquely rotted TRS etc cable will pass quite happily despite rubber hanging off :-) That is why visual inspection can be "everything" and why I take IR as a "well it tells me someone trapped a cable in a floorboard by badly done notching. The best tester is the RCD tester - Socket and See do some quite cheaply on Ebay, but I prefer Megger RCDT320. They can be rebadged by electrical factors and thus very cheap because "it doesn't say megger on it", but is the exact same machine.
If you have an earth rod, that requires a specialised tester - which it is useful to have (although cheaper to put a shed on wooden sleepers and not use Class-1 tools outside :-)
I bought one of these a while ago, and I used it to test all the wiring I did just prior to Part P. All the insulation and low ohms capability I needed.
I used a Robin tester for the rest.
Got a circuit for that?
A standard EL tester will do most earth rods ok - although it does not test the effect of long duration high current leakages. Having said that, the system RCDs ought to prevents those from occurring. In some ways you could argue that TT is safer with class I tools outside, since there is little chance of the earth taking on a potential significantly different from that of the local ground.
Indeed, I was checking around earlier tonight and it appears that most sparks are just using bog standard EFLI for TT domestic. Better to whack in another 4ft and use decent Furse etc.
That is the point - although there are not many class-1 tools. Still surprised they do not do 110V hedge trimmers, petrol is not ideal (ok electric is as noisy) and cordless too wimpy.
Peter Andrews pretended :
Is this for professional use?
The overview is that you use a high voltage source, rectify and more or less smooth it, through a resistor to limit current to a non-lethal amount, then feed it through a current meter (multimeter) and the insulation under test. A second meter monitors the test voltage, though this could be omitted for approximate tests.
An easy cheap source of high voltage is a microwave oven turntable motor. Hand cranked they produce about 1kV output. Note you need to use analogue meters with these, digitals cant handle the unsteady readings.
If you've got nothing better to hand and need to do a job there and then you can just use the mains to get 330v, or double with a basic capacitor/diode doubler for 650v, or triple it with a cockroft-walton for near 1kV. You can also charge up a few big mains lytics to get the required voltage.
The best option is a battery powered oscillator/transformer, which can give a steady regulated output at switchable voltages.
Be aware that these testers can produce lethal outputs. Be especially wary if using a directly mains powered version, it has additional risks due to lack of isolation.
_Do not_ use crude equipment like this if you dont properly understand the risks, if youre working professionally, if other people are within the area served by the wiring under test, or if you're tired or had a few last night.
NT
=20
Alternatively, you could do what I did, look on ebay for an old wind-up megger in a nice leather case complete with probes for about =C2=A310. Works a treat as long as there are no Prat-forms to be filled in. I can tell if it's under 0.1 ohms, or over 10 meg, but not exactly what it is, but then who cares (apart from Tony Prat-Prescot).
R.
Who would buy one? You can't drive a 110V down a long extension lead, so you'd be lugging around the transformer behind you. Then you've still got a 240V cable nearby to cut through. I think any conceivable benefits have evaporated, when an RCD as almost as effective and a damn sight more convenient.
It is, but it only needs to develop a few hundred V at a few mA across a high resistance. That's why there's such a differnce in price between it and those which also do continuity.
Owain
I think Harry B was asking me if it is for professional use!
The answer is no - just an additional DIY comfort factor and occasional fault finder (I had to borrow one recently).
Thanks everyone for your comments and advice - I shall ponder upon my requirements a little longer.
Peter
Not quite sure what you mean here. The one I got from TLC does both low ohms and also insulation tests....
Even better, BT test meters (the one with the 500V megohm test range). Fairly common on eBay and lighter than a wind-up.
I've also had far more failed wind-up meggers (insulation fails) than I have battery driven ones. Neither survives dropping, as the meter movement is always going to be fragile, but however well made a 50 year old wind-up looks, you're taking something of a gamble on it being usable.
for =A380?
Owain
This is the one that I was looking at:
Peter Andrews presented the following explanation :
Then it doesn't need to be 17th Edition, or calibrated, so any old Meggar will do - and much cheaper. I picked up a modern switched
250/500/1000 volt one, with low Ohms range for £15 (intended for my test bench) earlier in the year.HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.