Musician questions

it is a youtube video..

If a combo amplifier is used it is normal practice to earth the audio sockets' metal outer parts to prevent danger under fault conditions.  Screened audio leads connect from mains powered appliances like combo amplifiers to things like metal microphone stands or instruments. If the screen is earthed then it should prevent mains appearing on touchable metal parts due to a fault, shocking musicians. This has been confirmed by the O.P. in his recent tests. It is quite possible that the metal cover plate on the back of the bass amplifier is earthed as well, especially as it has mains rated things like a switch affixed to it.

Edit. Add.

DIY Tips - Electrical Safety for musicians on stage. (guitar-repairs.co.uk)

Z.

Some years ago I did the PAT at at a local church, the church warden said that the Church Organ Builder had requested that I “didn’t fiddle with the organ”, I replied that it was supplied by a transformer and blower next to the keyboard so I wasn’t even going into the organ loft, which apparently was the correct answer.

Class D I would expect to relate to the design of the amplifier circuit - it's one of the more efficient designs that most manufacturers use these days - as you say nothing to do with the equipment inspection and test.

‘Test 102’? - No idea……..perhaps that's a reference to a pre-programmed test on their equipment.

If you have continuity at a suitably low resistance value to the exposed conductive parts (especially the big metal panel on the back) you should be OK - and an appliance tester doing a class I test should pass it.  The panel may well be lacquered though or otherwise treated that makes getting a good connection between the earth probe of the tester and the actual metal tricky sometimes - which will give you a fail.

We should really all drop the ‘PAT’ term, as the ‘P’ isn't really applicable any more, but the full alternative of ISITEE likely won't catch on very fast either!

RichMcGill: 
 

The first reply said that the combo is class D with a switchmode supply

 

Am I right in thinking that class D with a switchmode supply has no impact on PAT?

 

The ‘Class D’ in the context of an amplifier, is related to the way that the main output devices are configured. This is an electronics term and has nothing to do with Class I or Class II etc with respect to the earthing regime. For the purposes of our discussion on this forum, its a red herring.

edit: the poster above posted while I was typing my response.

I suspect that “test 102” refers to one of the standard tests on something like a SEWARD PAT tester.  We have a SEAWARD HAL which is basically a bench mounted PAT machine which is intended for doing end-of-line production tests. It has a series of pre-defined test sequences & associated pass/fail levels. I suspect that the OEM has simply quoted the standard test number that he uses for that product.

Whilst you cannot really say you are PAT testing (Portable Appliance Testing Testing) you can have a PAT tester ( Portable Appliance Testing Tester) or be one.

The Portable bit does confuse the issue as well regarding what actually needs checking and testing,  in light of our renewed relationship with our antipodean cousins perhaps we should start saying “Test and tag”?

With most PAT courses being knocked out in a day, possibly finishing in time to take the exam on the same day as well, there’s never going to be enough time to discuss all the types of equipment and the issues you may find in real life, leaving people to learn on the hoof and gain experience by doing it.

I found some good Pat labels on EBay 

https://www.ebay.co.uk/itm/333205449634?hash=item4d9497aba2:g:S0YAAOSwv4hfKYZX

about as much use as a lot of supposedly electrical ‘PAT’ labels.

I suggest that looking for mechanical damage, chewed leads and signs of strain at the plug will get 90% of dangerous conditions.

I'd not necessarily  expect the audio jacks to be directly connected to mains earth, for reasons of hum loops they may be floating or audio transformer coupled,  but the panels you can touch probably are. 

If you have connection to some part of the case and the earth pin of the plug that is probably all the reassurance you will get in practice to treat as class I

Mike

mapj1: 
 

 

I'd not necessarily  expect the audio jacks to be directly connected to mains earth, for reasons of hum loops they may be floating or audio transformer coupled,  but the panels you can touch probably are.

 

 

 

I've been repairing (and playing) guitar amps for over 30 years … and a lot of them are connected to mains earth. 

Some other audio equipment too … but that's why DI boxes were invented … along with “balanced” signal and impedance matching.

The reason being the way the body acts as an antenna and interacts with the guitar strings … especially with single-coil pickups, but also (from experience) affects humbuckers.

May also surprise those not “in the know” that the strings (often via the bridge) are also connected to the “outer” of the jack socket.

It's also why many pedalboards have “isolated” mains supplies to pedals (in certain setups) … not only to help eliminate “earth loops”, but also simply because some pedals are “positive earth” and some “negative earth”. I made my own “DC power isolator” simply because of the cost of commercially available ones.

BTW, the earthing issue is why radio-link jacks are so popular with those that can afford a reasonable one … takes away some of the issues regarding the “human interaction” issue with the guitar.