Earthing protection stainless steel Vs Zinc Plate?  - Page 1
Forum section: electronics / projects

author: peps1
author_rank: Regular Contributor
timestamp: March 07, 2017, 09:05:24 am
content: I have an old piece of 1970s Japanese 120V audio equipment that has no earthing protection, It currently has the transformer and other electronics mounted on a steel zinc plated base (rest of the enclosure is painted steel), is this zinc plated base something I can just add a ground crimped wire to, or would re-fabricating the base in something like stainless steel be a safer option?

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: March 07, 2017, 09:20:44 am
content: Zinc will corrode and possibly pretty rapidly if there is any measurable current unless your crimp is also zinc. And then that's probably not wise either.But why did you want to earth it? Do you think it's inherently unsafe that you may want to create loops? Why not add a layer of insulation to the case?

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2017, 09:26:16 am
content: Quote from: peps1 on March 07, 2017, 09:05:24 amJust firmly bolt or screw a tinned eye terminal to it with your earth connection crimped.You can get bolt on push terminal lugs, that's another option.

author: peps1
author_rank: Regular Contributor
timestamp: March 07, 2017, 09:29:27 am
content: Quote from: RGB255_0_0 on March 07, 2017, 09:20:44 amI think anything mains powered that is not earthed is inherently unsafe

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2017, 09:40:44 am
content: Quote from: peps1 on March 07, 2017, 09:29:27 amMoreso if it's metal cased, today it's unlawful.

author: Gyro
author_rank: Super Contributor
timestamp: March 07, 2017, 09:41:58 am
content: You're going to have a real problem in a world that uses mostly Class II double insulated products then.Edit: ...many of which are metal chassis

author: Someone
author_rank: Super Contributor
timestamp: March 07, 2017, 10:07:10 am
content: Quote from: tautech on March 07, 2017, 09:40:44 amUnder which standard? Double insulated products can certainly have metal enclosures under all the standards I'm aware of.Quote from: peps1 on March 07, 2017, 09:05:24 amHome audio (hi-fi) is universally double insulated, otherwise you would need to have balanced connections between the components to avoid hum. Its possible to upgrade to that sort of interface but not worth the time or money. Its very unlikely that you need a ground, and from what you've said it would be safer to ask someone with an appliance test set such as an appliance electrician to just check its safety for you.

author: peps1
author_rank: Regular Contributor
timestamp: March 07, 2017, 10:15:56 am
content: Quote from: Someone on March 07, 2017, 10:07:10 amThis is not a Class II double insulated electrical appliance, its a Class 0, hence my concern.Quote from: Someone on March 07, 2017, 10:07:10 amIts not HiFi equipment, its an effects unit, and it is not double insulated, hence asking if I can Earth via zinc plated base.

author: NottheDan
author_rank: Frequent Contributor
timestamp: March 07, 2017, 10:24:20 am
content: Wouldn't earthing that possibly create a ground loop between that device and whatever it is connected to?

author: dmills
author_rank: Super Contributor
timestamp: March 07, 2017, 10:28:56 am
content: There are a couple of spare studs on that base plate that look like reasonable choices for the main earth wire, and I would add a jumper to the enclosure (Drill, grind off some of the interior paint, star washer, screw, nut), now what thread? There be a question.....Ring terminals are your friends here, also nylock nuts.Yea an earth loop is possible, but make it safe then make it quiet.Regards, Dan.

author: Gyro
author_rank: Super Contributor
timestamp: March 07, 2017, 10:33:14 am
content: Quote from: peps1 on March 07, 2017, 10:15:56 amAgreed, that isn't double insulated and needs to be earthed. I can't see where the ground wire goes (to the PCB?) but it needs to go directly to a chassis screw (with lockwasher). You really ought to have an earth jumper between there and top case too.P.S. I wasn't referring to your particuar issue or post - just responding to gross sweeping generalisations.Edit: Yes, those unused chassis studs look ideal - the technical requirement is that the earthing point can't also be a mechanical fixing, eg. a bolt holding down a transformer or connector.

author: peps1
author_rank: Regular Contributor
timestamp: March 07, 2017, 11:28:53 am
content: Quote from: NottheDan on March 07, 2017, 10:24:20 amYes, huge potential for a ground loop and Hum, but here at my home I have not had any issue A/B amps in the same outlet, so would think this would not be an issue (hope being the operative word)!Quote from: dmills on March 07, 2017, 10:28:56 amYep, whole bunch of M3 screws to slap a Earth Lug on and nylock down!Think the 6 mounting screws that connect the base to the lid will do the job, rather than grinding the lid and giving it its own jumper (just don't want to damage the case due to the value of it).Quote from: Gyro on March 07, 2017, 10:33:14 amthank you, that is good to know.

author: peps1
author_rank: Regular Contributor
timestamp: March 07, 2017, 03:46:10 pm
content: Thinking about the possibility of a ground loop...Would it be worth adding something like a bridge rectifier or parallel high current diodes with 1W resistor and a ~100nF capacitor section in there?Seems to be quite a common approach in audio amps to avoid hum and RF ripple.

author: Seekonk
author_rank: Super Contributor
timestamp: March 07, 2017, 04:09:12 pm
content: I assume you are something like 230V. I see a fuse in one side and a switch in the other. If you want safe, I'd add another fuse. That could be a slightly higher value so the chassis mounted fuse blows first in a normal fault. The other hidden inside for a line fault to ground. 1W resistor? I'd keep it a fairly high resistance if you want to prevent a ground loop. You have screw mounting to the transformer and isn't that the primary current fault?

author: peps1
author_rank: Regular Contributor
timestamp: March 07, 2017, 04:32:50 pm
content: Quote from: Seekonk on March 07, 2017, 04:09:12 pmSorry meant 10ohm/1W resistor, and yes there is a 0.2A fuse in the chassis.

author: Seekonk
author_rank: Super Contributor
timestamp: March 07, 2017, 04:39:29 pm
content: Say 100 ohm, and you need a second fuse on the switch line. Is a short happens on the switch side, what disconnects that short.

author: Gyro
author_rank: Super Contributor
timestamp: March 07, 2017, 06:30:58 pm
content: Quote from: Seekonk on March 07, 2017, 04:09:12 pmPersonally I would be looking to sleeve all of the primary side connections. Looking at the photo, there is absolutely nothing to prevent a signal wire coming loose from the front panel and slipping across onto the fuse, switch etc. There is a significant risk of equipment death even if the case is earthed.

author: julian1
author_rank: Frequent Contributor
timestamp: March 07, 2017, 06:43:28 pm
content: What voltage does the pcb need? Maybe replace the transformer/fuse and mains connector with a DC jack connected to the pcb - and power it all with an external wall-wart.

author: Someone
author_rank: Super Contributor
timestamp: March 07, 2017, 10:35:41 pm
content: Quote from: Gyro on March 07, 2017, 06:30:58 pmYep, time to upgrade this to a double insulated design with the appropriate cable management to make it safe.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 07, 2017, 11:00:01 pm
content: All of the recommended actions will make this safer. Probably it will be safe, as in "Meets current standards", and could be sold to someone else, both with a good conscience and legally. But not safe, as in "Can't possibly hurt anyone". The closest you can come to that involves putting it through a shredder (which is not a zero risk activity.)On the other hand, the possibility of making this safer doesn't imply a high risk of injury or death if modifications are not made. Even with the as is design it requires two problems to cause an injury. Depending on who and how this will be used it might be perfectly reasonable to leave it as is.

author: Someone
author_rank: Super Contributor
timestamp: March 08, 2017, 12:52:35 am
content: Quote from: CatalinaWOW on March 07, 2017, 11:00:01 pmWith the mains wiring touching LV and the case, it only requires a single fault in the insulation there to go hazardous (caused by a mains transient for instance). No way is it fully single fault protected.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2017, 02:27:40 am
content: While we are castigating it, what about the knot retaining the zipcord mains lead?It needs a three core mains lead with a proper cord grip running to a terminal block on the baseplate, the mains and low voltage wiring from/to the transformer secured to the case, and a bonding strap from the case to the baseplate. It also needs rewiring to put both fuse and switch in the live wire once a polarised plug is fitted, with the live goin to the bottom contact of the fuse holder.Unless the transformer is split bobbin, it can't be upgraded to meet the current standards for a double insulated appliance, though sleeving the mains wiring and heatshrinking over the switch and fuseholder terminals would help.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 08, 2017, 07:31:38 am
content: This would be extremely easy to modify to make it double insulated. Add sleeves to the few mains wires, possibly change the switch and fuse holder. Replace soldered connections with insulated crimp connectors.

author: Gyro
author_rank: Super Contributor
timestamp: March 08, 2017, 09:19:13 am
content: If you mean proper double insulated then it would probably need a transformer replacement too. A japanese one from the '70s probably isn't a split bobbin design and metal cover is in close proximity to the primary wiring.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 08, 2017, 02:58:17 pm
content: Quote from: Someone on March 08, 2017, 12:52:35 amA single fault puts mains on the case, but it still requires you to be touching to be in contact to a conductive path to something conductive at another potential. This second condition is not rare, so obviously it is not robustly double fault protected, but I can tell you from the personal experience of having discovered multiple devices with faults of the first type using only the detection devices built into all of humanity that it doesn't always cause injury.Safety is a continuum. Each person has different requirements for safety. I don't mind people who have high standards, just mind it when they try to enforce those standards on everyone. Here in the US the process goes like this. A hazard is identified. Something is done about the hazard. With that hazard eliminated (along with others of similar importance) there is now a new top hazard. Which "must" be eliminated. That path ends with everyone wrapped in foam padding and forbidden from most activities.To repeat my earlier post. If this equipment is to be passed on to anyone else who might have different expectations of its level of protection and a lesser understanding of the risks, it should be repaired/modified to current standards. If it remains in control of the OP, who understands the problems and can control the environment in which it is used, and who knows his own needs for safety, it may not require changes. He may have a well insulated shop floor, or may periodically check for a fault, or have some other strategy to bring the risks in line with his expectations.Clearly, others in this thread have strongly formed opinions about the risks they are willing to take. That is fine. In most cases those opinions are in line with what society currently expects, and that is excellent guidance for appropriate behavior for things that will go out of the OPs control.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2017, 03:36:57 pm
content: Would I plug it in as-is for my own use? Yes, after a basic HiPot test between the primary and the secondary. I might add an external earth strap using a ring terminal secured under the case screw nearest to the power lead, if I wanted to keep it original but be able to earth the chassis.Would I let anyone else use it as-is or leave it set up unsupervised in a family household? Hell NO. I've already listed the stuff to bring it inline with current safety standards.There's also the issue that its 120V - off a miswired autotransformer it could be extremely hazardous. Off a 55-0-55 portable power tool isolating transformer, it would be totally safe as-is. Maybe the answer is to just put a 110V 16A fused industrial plug on it with a 3A fuse in the plug, so it can only be plugged into suitable isolating transformers?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 08, 2017, 04:05:40 pm
content: Replacing the transformer would be very easy, too. I bet it's not too exotic of voltage, easily available with modern safety specs.