# What does it mean that a power supply is 'referenced to mains' and why is that dangerous?

I was watching a few different videos by BigClive on youtube. He was mentioning the danger of a particular light chain power supply because it was 'referenced to mains'. The voltage across the output terminals was 5v but an electrician's safety wand was still set off by being near the '5v' output.

Based on searching this site, it seems the neutral terminal of mains passed directly through to the power supply to be the neutral for the low voltage supply. Is this accurate? And how does this cause the detection wand to buzz?

• Pretty much. Dangerous because if you plug it in backwards (like you can on many American/EU sockets) your ground becomes the mains live terminal! – Tom Carpenter Feb 21 '17 at 16:59
• @TomCarpenter, the gadget shows East European mains connector, which does not have polarity, and can be plugged either way. There is no "live terminal" in the product, the whole theme is moot. – Ale..chenski Feb 21 '17 at 17:22
• I am a little familiar with isolation in power supplies and why it is a good idea. But how can the pins have a difference of 5v between them and still set off a detection wand? – OrangePeel52 Feb 21 '17 at 17:44
• Attach a battery to an overhead cable at a quarter of a million volts - it's still a 1.5 volt battery and if you were foolish enough to try and measure it you would confirm this. – Andy aka Feb 21 '17 at 17:48
• @Andyaka, can you give me an answer to that effect, and how it relates to 'referencing to mains'? I do understand that voltage is a measurement between 2 points, and not an absolute level, but that does not give me the answer to why a wand buzzes on this particular power supply. – OrangePeel52 Feb 21 '17 at 17:56

Most power supplies use a double-insulated transformer between the incoming mains and the outgoing extra low AC or DC voltage. This means that you will not get an electric shock if you touch one of the output terminals.

Some very cheap power supplies do not provide that isolation. One or other of the output terminal may be at the full mains voltage. In some cases, it might be safe (well, safeish) with the plug one way round, but dangerous with the plug reversed.

Such power supply designs should never be used for "wall warts". They should only be totally enclosed within the body of an appliance, and even then only if there are no exposed electrical parts (such as headphone sockets) that the user could touch.

• Simon, "Some very cheap power supplies do not provide that isolation"... Could you please provide an example of non-isolated AC-DC supply where terminals can be touched by user? If anything, such supplies cannot meet UL certification standard, and cannot be sold on consumer market. Any examples? – Ale..chenski Feb 21 '17 at 20:07
• @AliChen Since when did the people who buy $5 wall warts from random Internet sites even know that UL certification is a thing? Isn't it obvious a$5.00 wall wart must be twice as good in terms of "value for money" than a \$10.00 one? ;) – alephzero Feb 21 '17 at 20:55
• @AliChen Since Big Clive's been mentioned, see youtube.com/watch?v=QwqFkelUs_g – Simon B Feb 21 '17 at 23:50
• Simon, yes, I would agree that this particular lantern, as a PRODUCT, is life threatening when used outside its specifications - using a nonstandard USB cable with exposed power pins. Most mobile devices (phones, tablets) do not have conductive enclosures in direct contact with internal power or ground. – Ale..chenski Feb 22 '17 at 2:21
• @alephzero, people who buy UL-uncerified products from Internet will soon learn not to do this, at least their mourning relatives will. – Ale..chenski Feb 22 '17 at 3:04

"Mains isolation" is the approach used to make most power supplies safer. So the real question is "Why do we use mains isolation?", which has been asked several times on this site with some very high quality answers already -- see here, here, and here.

There are some safety concerns when not using mains isolation, but there are many products that use non-isolated supplies and are still quite safe. The specific example in the video is dangerous because it's not isolated and the power supply output is exposed to the users through the power cord.

• +1 for older references. However, as it can be seen at about 15:00 of the video, the cable end from AC-DC supply is a female-type end, covered with extended plastic shroud. How a user can be exposed then? – Ale..chenski Feb 22 '17 at 2:43
• @AliChen, the shrouded/female-type connector is certainly safer than many alternatives. But the issue here is the fact that the wires/cabling is accessible at all. This is a significant concern for UL certification (which you also mention in one of your other comments). This product probably doesn't care about UL at all, but it's much easier for these types of supplies to fail in a way that leaves the output at mains voltage levels than the same happening to isolated supplies. So users may incorrectly assume that the plug is safe to handle just like any other USB/low-voltage plug. – youtooth Feb 23 '17 at 1:11

A power supply has a pair of connections for mains input (it may also have a connection for mains earth). While one of these connections is nominally neutral portable appliance standards generally assume that both of them are potentially dangerous. Reasons for this include the potential unreliability of flexible wiring and frequently plugged connectors and the fact that some countries use unpolaised sockets.

"refrenced to mains" means that there is a low impedance connection between the input connections and the output such that hazardous voltages/currents* on the input may be passed to the output. So the output terminals are a potential shock hazard. Such power supplies are cheaper to build as no transformer is needed.

It is possible to design equipment with transformerless power supplies safely but it means treating anything connected to the output of the power supply as-if it was connected to live mains.

I doubt that the cables and connectors on those lights would pass muster as mains connectors at least in Europe. The lights themselves it's hard to tell as it depends very much on how thick the plastic is.

User expectation is also an issue. Safety standards often talk about "reasonablly forseeable misuse". If something looks like a typical wall wart then people are going to assume it is a typical wall wart. Even if the equipment supplied with the non-isolated supply meets insulation and touchproofing standards for mains it would be all too easy for the wall wart to get repurposed to supply something else.

* to shock someone requires both sufficient open circuit voltage and sufficient short circuit current.

The 5V DC it outputs is relative. Between DC + and DC -, it certainly is safe. But between DC output and earth of your house, there is either full mains voltage or nothing depending on how the lamp is connected.

If the lamp is referenced to neutral, it's safe. But if it's referenced to live and neutral is connected to ground, it's not safe. Just to make it more interesting, many countries have non-polarized sockets so you can't be sure which terminal is live and which is neutral.

Here's an illustration of one of the cases: