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 doesnât happen often. Iâd 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.
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).
Depends. IIRC IEC 62368 says that all accessible metalwork needs to be protected. Are the RCAs accessible ? The phrase âof courseâ 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 didnât have in mind fixed installations. People have taken to using it for other stuff. But outside the workplace itâs voluntary of course.
I just looked it up. Itâs 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 itâll be dissipating 4.5W and since itâs getting rid of that, presumaby to the free air around it, I imagine itâll already be well above 25C.
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 donât 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 donât see the issue using a cl60 like this so will continue to do so, since I bought a big bag . 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 wasnât 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.
Weâre back to the âasymmetry of riskâ problem.
From the ownerâs point of view the risk of leaving part of his equipment (the signal ground) unprotected is negligible. So he doesnât 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 heâs concerned itâs never going to happen.
If, on the other hand, youâre the manufacturer or the safety regulator and youâre looking at 10,000,000 phone chargers in peopleâs homes then a 1 in 100,000 per year risk is 100 people getting a shock each year. Thatâs not at all negligible. Thatâs 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 Iâm concerned, is that a bridge like this https://uk.farnell.com/vishay/pb3510-e3-45/rectifier-bridge-1000v-35a-pb/dp/1815637 costs a couple of quid and if itâs 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 firmâs radios (actually forget that - no taxi firms use radios any more) and youâve got a bomb-proof earth lifter for less than the cost of the ampâs 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 donât 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 tend to use a higher value resistor and lower value capacitor, really to knock any circulating current on the head, but otherwise itâs the way I go too.
If the noise were to be really high amplitude you could short the bridgeâs + 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 thereâs 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.
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.
â 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?â
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.
Might get a few made if anyoneâs interested. Itâs 37x37mm and includes connections for a switch to bypass the lift. Iâve also put additonal holes for various caps and resistors.
. As mentioned all the commercial First Watt offerings are built like this.
