.....why 2 buzz testers show short and 1 buzz and 1 I R tester show clear.
The reason you get different results from different testers is firstly because they're actually performing completely different tests. The IR tester is using a very high test voltage on a dead circuit whereas the volt testers are not using any test voltage on a live circuit.
Secondly the characteristics of the different voltage or continuity testers aren't the same. There will be a difference in their sensitivity and a difference in their input impedance so depending exactly what you're testing they'll often show different results.
Finally you need to use the right tester in the right way and test for the right thing.
Here's an example;
If an RCD is tripping occasionally and I suspect it's being cause by a cooker. Here's some tests I could do.
1. I could connect an earth leakage clamp meter around the cpc in the isolator and take a reading with the cooker on.
2. I could disconnect the cooker and use a continuity tester on the Mohm scale to test the resistance between the L+E and the N+E.
3. I could disconnect the cpc whilst the cooker is on and use a voltmeter with one probe on a known good earth to see if leakage is causing a voltage on the CPC.
4. I could use an earth leakage clamp meter around the L+N simultaneously at the isolator whilst the cooker is on.
5. I could disconnect the cooker and use a mega tester to test L to E and N to E.
All these tests will show some of the effects or the possible potential of the cooker to produce a leakage current to earth but only one of them will definitely give you an accurate indication of the
actual earth leakage being detected by the RCD.
Like I said it's about using the right equipment in the right way and understanding exactly what the reading it's giving you means.