Ground loops - boat vs lab style isolation transformer and secondary PE - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: March 31, 2018, 04:46:27 am
content: From reading on the web there appear to be 3 basic types of 1:1 mains isolation transformers. Please correct me if I'm wrong.1) Hospital - mains protective earth (PE) is connected directly through to the PE on the output socket. There is no electrical connection between the secondary and the PE (not sure about this).2) Lab - There is no PE connection to the output socket.3) Shore to Boat - The boat side secondary neutral is connected to the boat side PE which is electrically isolated from the shore side PE to prevent galvanic corrosion.If this is correct, why don't lab supplies use the shore to boat style connections where the secondary neutral is connected to a PE wire that is isolated from the primary PE. Presumably this would allow things like 3 terminal mains interference filters to function as designed (or does it not matter) while still allowing you to break a ground loop when doing audio level type measurements in low voltage circuits.Is a lab style power isolation transformer any less safe than a boat style isolation transformer when powering a device that expects a PE connection on the power socket, for example a guitar amplifier where the signal ground is connected to the metal chassis that is supposed to connect to PE?

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 09:04:14 am
content: The problem with using a boat style transformer is, there's still a risk of shock when touching the live, because the neutral is connected to earth. The whole point of a lab isolation transformer is to isolate the neutral connection from earth, so just touching the live connection on its own doesn't result in a shock. It also enables the input of a piece of test equipment, such as an oscilloscope to be connected to the mains, without causing a short circuit.

author: Ian.M
author_rank: Super Contributor
timestamp: March 31, 2018, 09:53:06 am
content: +1Isolation transformers used on boat shore-power feeds aren't there for human safety, they are there to allow the boat's local ground to be independent of the shore power distribution system ground to minimise electrolytic corrosion of the boat's underwater metalwork.Without an isolating transformer, and with the mains supply ground bonded to the boat's DC system 0V (battery negative) to create an equipotential zone for safety, the bonded underwater metalwork (usually the sterngear, but can include other underwater fittings) is effectively connected to every other boat in the marina that also doesn't have an isolating transformer. All it takes is ONE boat with a broken ground strap from its engine block to its batteries, and the mains supply ground bonded to 0V on the battery side of the break, and the engine block will go positive due to current through sensors and other engine electronics (and the Alternator is usually permanantly powered unless a manual battery isolator is used) it will put DC current into the water via the prop shaft. The owner of the defective boat doesn't notice any problem apart from his engine not working, as his underwater metalwork is the positive electrode so doesn't corrode significantly, but the other boats become negative electrodes, and his slip neighbours will experience rapid (and expensive) corrosion of their underwater metalwork.Quote from: Hero999 on March 31, 2018, 09:04:14 amHowever you *MUST* be aware that as soon as you connect a test equipment ground clip to any part of the D.U.T. (device under test) on the line side of any isolation barrier in its PSU, you have defeated the isolation, and all line side parts of that PSU except the node you've just grounded are now hazardous. Also the isolation transformer prevents any upstream GFCI (RCD) protecting any circuit downstream of it, and if wired for testbench use, also prevents any downstream GFCI from operating.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 31, 2018, 10:28:08 am
content: Quote from: e100 on March 31, 2018, 04:46:27 amAFAIK -Usually- the transformersecondaryconfirmed and polarity correct 'Neutral' line is tied to the bonded earth/ground coming in and out, with the metal chassis and transformer body/Faraday shield wire etc .so it complies with wiring regulations and any connected 10m or 30ma GFCI / RCD trips on that secondary output if there's a faultIt also reduces or eliminates line noise and garbage from the primary, even though it's not lab style 'isolated/floating' soBANG! Cityis imminentGreat for protecting gear, reducing line generated crap, and have a working GFCI /RCD on the secondary output that is isolated from the main switchboard GFCI / RCDi.e. good for hospitals and general purpose stuff,no good for isolation activities on the test bench,unless you feel lucky and think you can get the correct wiring combo going every time...without a direct 120/240v short

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 01:09:06 pm
content: Quote from: Ian.M on March 31, 2018, 09:53:06 amActually it's the anode which corrodes, not the cathode and it has nothing to do with the boat's DC electrical system.The problem is, without an isolation transformer, to ensure the boat and land are at the same potential, the boat's hull (usually metallic) and engine has to be connected to the mains earth, an electrode buried somewhere on land. Now a galvanic cell is formed between the mains electrode and the boat's hull and engine, with the electrolyte being the water.Using an isolation transformer, means the boat's hull and engine no longer need to be bonded to the mains earth. Instead they're connected to the neutral conductor of the isolation transformer,'s secondary so the boat's entire electrical system is isolated from the electrical system on land. This breaks the galvanic cell's circuit, so there's no longer a DC current path and no galvanic corrosion.Quote from: blueskull on March 31, 2018, 09:57:08 amI can understand why you feel uncomfortable with an isolation transformer: the secondary can become earthed without warning, but it's really no more dangerous, than a GFCI.In many cases isolation transformers are preferred, over GFCIs because they're passive and have no moving parts. A GFCI can only trip a limited number of times and it's theoretically possible for the contacts to weld and it fail to open. This is why isolation transformers are used in hospitals. Another factor is a GFCI tripping could cut the power to life support machines, so isolation transformers are preferred for their reliability.In my opinion an isolation transformer, with an insulation monitoring device, which gives a warning if the secondary is earthed, is the best option and is what's commonly used in hospitals.

author: C
author_rank: Super Contributor
timestamp: March 31, 2018, 02:12:41 pm
content: As the Boat's power system gets larger, so does the safety prevention.Safety is important and salt air causes big problems.They often have meters that tell all the facts.They do not wait for a GFI to trip before turning off something bad.Where Lan.B talked about water forming a path. They use the boat's metalwork to sense a fault in the electrical safety ground on the boat.

author: e100
author_rank: Frequent Contributor
timestamp: March 31, 2018, 03:04:00 pm
content: Quote from: Electro Detective on March 31, 2018, 10:28:08 amI'm a little confused, this pagehttp://www.epanorama.net/phpBB3/viewtopic.php?t=132969kind of implies that some hospital isolation transformers have floating outputs, and some don't.

author: e100
author_rank: Frequent Contributor
timestamp: March 31, 2018, 03:15:48 pm
content: If you were doing audio recording and had a ground loop hum problem due to two devices having their signal grounds connected to protective earth (as is typical with electric guitar amplifiers), and you didn't have an audio isolation transformer to hand, would it be acceptable from a safety or regulatory point of view to use a mains lab or boat style isolation transformer to power one of the devices in order to break the ground loop?

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2018, 07:06:37 pm
content: Quote from: e100 on March 31, 2018, 04:46:27 amThey do not because this would defeat the purpose of using the isolation transformer for safe neutral referenced measurements. What is needed is a single point ground at the point where the measurement is made and if the ground and neutral were connected at the isolation transformer output, then it would create the very ground loop that the isolation transformer was used to prevent.Another way of saying this is that the lab isolation transformer allows moving the ground to neutral connection to a point which allows the measurement without the neutral (or hot) current to interfere. Usually this point is through the test instrument's ground connection although that is of questionable safety. Adding a nice heavy ground strap to the measurement point is sufficient.Aground isolation monitorcan be used with an isolation transformer to do this with increased safety.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 31, 2018, 09:39:20 pm
content: Quote from: e100 on March 31, 2018, 03:04:00 pmIt depends on what device/s and apps the hospital is using,if they want 'medical isolation' so the GFCI/RCD works on the secondary side, and line interference suppressed from going into their devices (and or coming back out), then the earth ground must be tied with the secondary Neutral only,Quote from your link:"There are quite a variety of mains isolation transformers designed for different uses. Those mains isolation transformers all isolate the main live and neutral wires,but vary what they do to the ground connections.Some transformers cut the ground connection to mains ground, some other does not.In some applications the secondary is left "floating" and on some other applications on end of it is connected to ground. "I have a few in the different wiring flavors for different applications, all labelled appropriately and with diagrams to avoid any confusionSome have center taps and wires everywhere plus Faraday Shield and all sorts of unlabelled weird links,plus my favorite 'Friday assembly' loose connections and poor earth ground bonding, and internally broken strands on NEW WIRE to really get you thinking...+Most recent products like this MUST be opened and checked if you want peace of mind and performancethe quality at any price is suspect nowadays