LISN device and isolation transformer - Page 1
Forum section: electronics / testgear

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 21, 2017, 06:33:11 pm
content: HiI bought a LISN unit but when I plug it in it trips the main fusebox. I think this is to do with currents through the Earth path. One suggestion is an isolation transformer. Anyone got any other. I heard dealing with isolation transformers can be very dangerous.ThanksTrev

author: Hydron
author_rank: Super Contributor
timestamp: October 21, 2017, 06:41:56 pm
content: Mains LISNs let a HUGE leakage current flow (as an unavoidable consequence of the design specification) - on the order of hundreds of milliamperes to almost an amp. This is guaranteed to trip any RCD, and can be extremely dangerous if not correctly setup.You'll need to be very careful about equipment setup and protective earthing, and will likely need an isolation transformer. Refer to manufacturer guidelines about how to connect the LISN safely - as an example Tekbox has their manual online here:https://www.tekbox.net/images/documents/testequipment/tblc08/TBLC08_Manual.pdfThe circuit diagram is prescribed by the testing standards so is readily available (including in the above manual) - have a look at the big caps from phase to earth and you'll see why they are devices which require care to use!

author: coppice
author_rank: Super Contributor
timestamp: October 21, 2017, 08:03:51 pm
content: Quote from: trevwhite on October 21, 2017, 06:33:11 pmYou might well want an isolation transformer so that you can connect test equipment to the load without something going bang. However, this doesn't help with the problem of your RCD tripping. As Hydron said, you can have a considerable current to earth, through the large capacitors in the LISN. Any RCD that won't pop on those won't provide much of a safety benefit - no protection against death through shocks, and limited protection against fire hazards. You have to live with that, remove any RCDs, and take care.I guess it should be possible to put an RCD between the LISN and the load, so you are safer while working on that load, although you are adding additional inductors if its an electromagnetic RCD. I've never seen anyone do this.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 21, 2017, 08:33:27 pm
content: Thanks for the replies. Safety is obviously important and appreciate comments. I need to get a setup I feel confident of.Sent from my SM-G950F using Tapatalk

author: Hydron
author_rank: Super Contributor
timestamp: October 21, 2017, 09:03:00 pm
content: The isolation transformer is to put between the RCD and the LISN input, which will stop it tripping, as it stops the RCD from seeing the leakage current. This is where isolation transformers normally are used incorrectly (bypassing the RCD decreases safety) but in this case there is no choice.The isolation transformer is NOT there to let you float any test equipment or for any safety purpose, and the earth connections must be maintained throughout (indeed they are of critical importance for safety and measurement integrity).The equipment you are testing should be connected directly to the LISN mains output as specified in the EMC testing standards, with no RCD etc. You shouldn't really be working on the equipment under test while connected to the LISN other than for testing normal operation - certainly don't do anything where you'd be wanting the protection of an RCD (this is a good rule in general, relying on one tripping to save yourself isn't smart - they're there for a last line of defence backup only!).Sorry to go on about it, but this stuff is super important for safety. A LISN is an unusual piece of test gear in that it creates serious safety issues if used incorrectly. At the very least, as well as following the manual instructions for setup, all protective earth connections on the LISN and the device to be tested (if applicable) should be tested for a good solid connection to mains protective earth before powering anything up. The RCD tripping is actually a good sign, it's doing what it should and also shows that the earth connection is there.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 21, 2017, 09:06:32 pm
content: Quote from: Hydron on October 21, 2017, 09:03:00 pmThanks. More detail the better on this subject I think.Sent from my SM-G950F using Tapatalk

author: coppice
author_rank: Super Contributor
timestamp: October 21, 2017, 09:25:32 pm
content: Quote from: Hydron on October 21, 2017, 09:03:00 pmSince you have defeated the effect of the RCD with this configuration, there is really no benefit in it being there.

author: Hydron
author_rank: Super Contributor
timestamp: October 21, 2017, 10:14:13 pm
content: Quote from: coppice on October 21, 2017, 09:25:32 pmAgreed, it's best not to have it - the OP mentioned tripping the fusebox though so my assumption was that the RCD is there whether you like it or not (i.e. it's in the fuse-box and can't be disabled).

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 21, 2017, 10:19:42 pm
content: Quote from: Hydron on October 21, 2017, 10:14:13 pmYep, I can't remove it. I think the isolation transformer is my only option. I jnow need to ensure best safety practices when using one .Sent from my SM-G950F using Tapatalk

author: Hydron
author_rank: Super Contributor
timestamp: October 22, 2017, 11:49:11 am
content: With fuse-box RCDs on everything in the UK, it makes one wonder why we can't do away with the dirty gross pull-cord switches in bathrooms.I've got experience of bathroom electrical safety codes in another western country, and it boils down to: keep stuff away from the splash zone, and anything that can be reached from inside the bath/shower needs special protection or has to run on low voltage. No pull-cord bullshit required./rant

author: coppice
author_rank: Super Contributor
timestamp: October 22, 2017, 12:16:54 pm
content: Quote from: Hydron on October 22, 2017, 11:49:11 amWith conventional light switches I suspect there would be a lot of RCD trips. However, offering bathroom rated light switches, with a silicone membrane inside, and relying on RCD to handle failures in those membranes, seems reasonable.

author: Z_Peter
author_rank: Newbie
timestamp: January 13, 2022, 09:12:23 am
content: Thanks for all the good advices here. One question regarding isolation transformer and test equipment - LISN is ofc connected behind it with earth connected to the horizontal plane, but how should be the EMI receiver connected with regards to it's mains input? Normally I would use a socket strip behind isolation transformer and supply everything from there, but I saw some videos/drawing suggestion otherwise.Thanks

author: coppercone2
author_rank: Super Contributor
timestamp: January 13, 2022, 03:23:21 pm
content: You can put a large transformer to limit inrush current (grounded).Rather then messing around with isolation and stuff, I recommend you rebreaker the panel to have a higher interrupt current GFIC.There are off the shelf units that do a higher currenthttps://iaeimagazine.org/features/now-that-industrial-gfcis-are-here-inspectors-have-a-proactive-option-for-shock-protection/and adjustable unitshttps://m.littelfuse.com/~/media/protection-relays/brochures/littelfuse_sb6100_industrial_shockblock_brochure.pdfA LISN is advanced it would not hurt to get the correct equipment to work with it. Industrial stuff deals with high leakage all the time (the equipment is just not there yet).You can make a box with the GFIC that connects to a non protected circuit (piggy back it to lisn) or make a special outlet (recommend a different color and a engraving in it that tells the users that the trip current is less and the outlet is less safe).If you have a LISN and the equipment needed to read it, you can certainly get the correct breaker instead of fooling around. The prototypes that are being tested are likely extra dangerous. Of course you need special precautions, since the bathroom GFIC is going to protect you better when you are moist, so be sure to be dry/insulated when you work with a higher leakage GFIC.