DIY Help

It probably is - earth lifters usually are. But ones based on a bloody enormous pair of anti-parallel rectifiers always feel safer to me. I one read somewhere that because of their lower mains voltage Americans worry almost entirely about burning their houses down with electricity (which to be honest probably kills more people) while Europeans in general and Brits in particular worry about electric shock. The major difference between them is timesacle. If it takes the thermistor a few seconds to heat up to fuse-blowing temperature well so be it - fires take ages to start. But shock can kill you in under a second at the end of which a thermistor might be barely warm.


Worst case fault is that amateur soldering of the mains live wire to the fuseholder causes the wire to come adrift when the amp is jostled, say by someone pulling a signal lead out (one good reason for buying an IEC inlet with a combined fuseholder). The loose live wire could then touch anything. If the fuseholder is a separate one and is mounted on the back of the amp, close to the phono sockets, then the probably uninsulated phono ground wiring is quite a good candidate for first-contact. Which will tingle a bit when you next try to plug something back into it.

As I say, it doesn’t happen often. I’d be more worried that the PAT test would blow the thermistor o/c. But that would be fail-safe, in that it would fail the PAT test.


What is the PAT test current?

The CL60 is rated at 32 amps(240v) . its 10 ohm nominal, but drops quickly up to 5 amps.

Depends. The tough test is 25A for 5-10s. But there is an optional lower-current version for electronic equipment. I think my tester does this at ~7A but you can get them which go down to under 1A. The common spec is for an earth continuity resistance below 0.1ohm and that gets tricky to measure at very low currents (also you want your safety earth not to go pop before the fuse does).


Well its 32amp so its fine then :slight_smile:

Would they probe the RCA ground as per course? Some gear doesn’t even have that connected to ground.

Depends. IIRC IEC 62368 says that all accessible metalwork needs to be protected. Are the RCAs accessible ? The phrase “of course” springs to mind.

That said, you could ask whether PAT testing is appropriate for hi-fi gear at all. It was designed to test PORTABLE equipment and at the start they fairly clearly had in mind power tools, maybe hand-held kitchen appliances, powered cleaning equipment etc. They didn’t have in mind fixed installations. People have taken to using it for other stuff. But outside the workplace it’s voluntary of course.


I just looked it up. It’s only 32A for as long as you can keep its temperature at (or below, I guess) 25C. The max rated steady-state current is 5A. At 5A and 0.18ohm it’ll be dissipating 4.5W and since it’s getting rid of that, presumaby to the free air around it, I imagine it’ll already be well above 25C.


I know very little about PAT testing.

My point about the 32amp was assuming the PAT test is rather short term. But is it really a circumstance that would arise? I just don’t see it.

My WAD EL34 amp has a 10 ohm 1/2w resistor to lift audio ground, as per the schematic. Several other commercial amps I have had, especially PA amps have lifted grounds or switches.

I can see my AV amplifier reacting well to 24 amps being dumped through the RCA ground.

I really just don’t see the issue using a cl60 like this so will continue to do so, since I bought a big bag :grin:. As mentioned all the commercial First Watt offerings are built like this.

My main point of course is we seemed to have an OP who wasn’t even sure where to hook up transformer secondaries without confirmation. This worried me a bit.

Anyone working with mains voltage should be able to understand the schematic and weigh up the risks of using a cl60 ground lift themselves.

We’re back to the ‘asymmetry of risk’ problem.

From the owner’s point of view the risk of leaving part of his equipment (the signal ground) unprotected is negligible. So he doesn’t care that in the event of an accident the weakest link is a thermistor, or a 1/2W resistor, which will provably fail before the protection (the fuse) kicks in. If the risk of this happening is 1 in 100,000 per year then as far as he’s concerned it’s never going to happen.

If, on the other hand, you’re the manufacturer or the safety regulator and you’re looking at 10,000,000 phone chargers in people’s homes then a 1 in 100,000 per year risk is 100 people getting a shock each year. That’s not at all negligible. That’s unacceptable. The regulators write the rules, so the rules are demanding. They require all the accessible metalwork to be protected.

What swings it for the regulator, as far as I’m concerned, is that a bridge like this costs a couple of quid and if it’s wired with the diodes in anti-parallel would sustain hundreds of amps under fault conditions for long enough to see any realistic fuse off. Put a sacrificial resistor across it to maintain signal ground at 0V and maybe a few nF of capacitance in case it occasionally picks up the local taxi firm’s radios (actually forget that - no taxi firms use radios any more) and you’ve got a bomb-proof earth lifter for less than the cost of the amp’s knobs.

I note your list of people who think the safety earth is a nuisance. I can raise you one or two manufacturers who fit a three-pin IEC inlet and then simply don’t connect its earth terminal to anything at all. And there are others who just connect it to an interwinding screen on the mains transformer. And then they CE mark the product.


I wasn’t saying there weren’t better ways to do it :slight_smile:

I am clearly not as risk averse :grin:


Rod Eliott has quite a useful discussion about it, and includes this, which aligns with @Valvebloke 's description

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That’s what I use.

I tend to use a higher value resistor and lower value capacitor, really to knock any circulating current on the head, but otherwise it’s the way I go too.

If the noise were to be really high amplitude you could short the bridge’s + to - as in the diagram but make the Earth and signal ground connections to the two ~ nodes. This doubles the voltage at which the diode assembly starts to conduct. The price you pay is that there’s now no redundancy if any one of the diodes fails. Then again, Rod asserts that they always fail short circuit, in which case it would be fail-safe.


it is also what I use, plus a bypass switch for earth/float option for the audio grnd

Just to let you know that i raised the question of dangerous earthing in the Diyaudio for this circuit because i am worried that they are using just a thermistor in case of a blow up. They have responded that the circuit IS potentially dangerous. I cannot simply understand how people could sell stuff like this to DIYers. It should be made illegal.

Sell what? It’s how nelson does it In his commercial offering but is in no way part of the PCB, the grounding scheme is entirely up to you.

If they sell the PCBs and produce videos on how to build the thing, safety considerations should be paramount.

I’d consider it safe. Clearly some don’t.

You are building something with mains voltage, the safest thing would just to not publish or allow anyone to go near it.

“ I am abit nervous about the grounding of this circuit. If something bad happens, would the person touching the chassis be dead long before the circuit with the thermosistor CL60 becomes active?”

From your diyaudio post.

Just to clarify the chassis is connected directly to earth. Should the chassis become live the fuse/rcd will trip.

The cl60 is on the audio circuit, the risk is if the audio circuit/ground goes live but not the chassis, that current would possibly blow the cl60 or may not blow The fuse, should you then touch the audio ground like the rca you could get a zap.

If you are worried about that then use the above rectifier solution suggested.

I’ve been playing with a board design to implement the bridge-based solution:

Might get a few made if anyone’s interested. It’s 37x37mm and includes connections for a switch to bypass the lift. I’ve also put additonal holes for various caps and resistors.

Bridge is:

Happy to supply a few in exchange for a small donation to the forum.