Author Topic: LISN device and isolation transformer  (Read 1251 times)

0 Members and 1 Guest are viewing this topic.

Offline trevwhite

  • Frequent Contributor
  • **
  • Posts: 760
  • Country: gb
LISN device and isolation transformer
« on: October 22, 2017, 05:33:11 am »
Hi

I 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.

Thanks

Trev
 

Offline Hydron

  • Frequent Contributor
  • **
  • Posts: 375
  • Country: gb
Re: LISN device and isolation transformer
« Reply #1 on: October 22, 2017, 05:41:56 am »
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.pdf

The 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!
 

Online coppice

  • Super Contributor
  • ***
  • Posts: 4203
  • Country: gb
Re: LISN device and isolation transformer
« Reply #2 on: October 22, 2017, 07:03:51 am »
I 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.
You 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.
 

Offline trevwhite

  • Frequent Contributor
  • **
  • Posts: 760
  • Country: gb
Re: LISN device and isolation transformer
« Reply #3 on: October 22, 2017, 07:33:27 am »
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

 

Offline Hydron

  • Frequent Contributor
  • **
  • Posts: 375
  • Country: gb
Re: LISN device and isolation transformer
« Reply #4 on: October 22, 2017, 08:03:00 am »
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.
 

Offline trevwhite

  • Frequent Contributor
  • **
  • Posts: 760
  • Country: gb
Re: LISN device and isolation transformer
« Reply #5 on: October 22, 2017, 08:06:32 am »
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.
Thanks. More detail the better on this subject I think.

Sent from my SM-G950F using Tapatalk

 

Online coppice

  • Super Contributor
  • ***
  • Posts: 4203
  • Country: gb
Re: LISN device and isolation transformer
« Reply #6 on: October 22, 2017, 08:25:32 am »
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.
Since you have defeated the effect of the RCD with this configuration, there is really no benefit in it being there.
 

Offline Hydron

  • Frequent Contributor
  • **
  • Posts: 375
  • Country: gb
Re: LISN device and isolation transformer
« Reply #7 on: October 22, 2017, 09:14:13 am »
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.
Since you have defeated the effect of the RCD with this configuration, there is really no benefit in it being there.
Agreed, 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).
 

Offline trevwhite

  • Frequent Contributor
  • **
  • Posts: 760
  • Country: gb
Re: LISN device and isolation transformer
« Reply #8 on: October 22, 2017, 09:19:42 am »
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.
Since you have defeated the effect of the RCD with this configuration, there is really no benefit in it being there.
Agreed, 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).
Yep,  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

 

Offline Hydron

  • Frequent Contributor
  • **
  • Posts: 375
  • Country: gb
Re: LISN device and isolation transformer
« Reply #9 on: October 22, 2017, 10:49:11 pm »
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
« Last Edit: October 22, 2017, 10:51:07 pm by Hydron »
 

Online coppice

  • Super Contributor
  • ***
  • Posts: 4203
  • Country: gb
Re: LISN device and isolation transformer
« Reply #10 on: October 22, 2017, 11:16:54 pm »
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
With 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.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf