How safe is 48V DC?

Question

In many Power over Ethernet (POE) setups the transmission voltage is 48V or slightly more. While higher voltage has obvious efficiency advantages, how safe it is? Is there any risk of electrocution when accidentally exposed, in particular to children? Such wirings lack the protection that is used for 120/230V, and frankly the difference between 48V and 120V doesn't seem to be that significant.

Answer 1

frankly the difference between 48V and 120V doesn't seem to be that significant.

120V is 2.4 times higher than 48V - hardly what I would call 'not that significant'. 120VAC is even worse, for two reasons:-

1. 120VAC has a peak voltage of 170V, 3.5 times higher than 48VDC.

2. The 'electric shock' feeling occurs on every peak of the AC waveform, whereas with DC it mostly occurs on initial contact.

It only takes about 30mA of 60Hz AC current through the heart to cause fibrillation, compared to 300-500mA of DC current. The 'let go' current (above which you cannot let go of a grasped conductor) is 4 times higher for DC than AC. So that means you need 4-17 times more DC voltage to get a fatal shock.

Combine 2.4 times higher voltage with 4-17 times higher susceptibility, and 120VAC is approximately 10-40 times more deadly than 48VDC.

But is 48VDC safe in an Ethernet cable? Provided you don't strip off the insulation and poke the bare wires into your flesh, the chances of getting a fatal electric shock from it are negligible. I know from personal experience because I was a telephone exchange technician for 15 years, and regularly worked on live equipment with exposed contacts. The biggest DC shock I ever got was a light tingle when grasping a 50V bus bar (90VAC ringing was a different story...).

Answer 2

48V is the practical and LEGAL definition of the maximum voltage to be considered "low voltage" and intrinsically "safe". Certainly 48V delivered UNDER your relatively insulating skin surface could kill you if delivered in the "right" place. But we are assuming people aren't walking around with subcutaneous electrodes exposed to accidental contact with "LV" wiring. 48V is reasonably safe for most people under normal conditions.

As observed by @crowie, the original (and current) technology wired public telephone system operates on 48V for over 100 years. That old telephone technology is very likely the precedent for establishing 48V as the legal limit.

Answer 3

No death by electrocution going to happen

48V is considered "safe", and that is for good reason.

First, the impedance of the human body at 50V is around 45kΩ (though measured on adults). While children are overall smaller and thus should have slightly lower impedance, it's the skin resistance which makes up 95% of that impedance (internal body fluids are pretty good conductors), so the size doesn't matter all that much.

(Note how body impedance is a funny thing, it goes down rapidly as voltage goes up, at 240V it's 10-15 times lower!)

Further, an electric current needs somewhere to go, obviously. No closed circuit, no current. That's why birds sitting on an overland line aren't fried.

These 48V are 48V against ground. In all likelihood, the next closest thing to "ground" that you have contact with is "parquet/laminate" or "floor tiles" or something similar, in other words, resistance around infinite, current zero.
Even touching the hot wire on 240V has a good chance of "not much bad stuff happening", if you wear shoes and aren't precisely standing in a puddle of water (although for obvious reasons I wouldn't advise trying your luck!).

Let's assume the absolutely worst case: a child puts one finger onto the ground pin on a wall plug, and sucks on the PoE cable (looks edible, doesn't it!). Against all odds, the PSE is broken or heftily non-compliant and instead of supplying 10.2V/4mA max as per default, it supplies full operational voltage, and unlimited current. Or, it takes some random pattern that the child accidentially made for a valid negotiation, whatever.
Also, for an unexplainable reason, the current doesn't short circuit over the data wires (the likely thing to happen would be exactly that, a little spark on the child's tongue, and the child dropping the cable in a fright).
Let's just say there's actually 40V on the wire, and the current "decides" to go through the child's body, against all reason and against the laws of physics.

Cable-in-mouth will eliminate one skin barrier and thus approximately halve the body impedance. That's 22.5kΩ remaining. Let's round down to 20kΩ to be sure. No, you know what, let's be outrageous, and say 10kΩ. 48V/10kΩ = 4.8mA.
Which... is harmless even for alternating current. It takes about 8-10 times as much alternating current (of a frequency in the critical 50-60Hz range) to stop the heart.

Now, on top of that, PoE doesn't have alternating current, it's DC. So the scary bit about cardiac arrest doesn't even apply.

Of course, DC can in principle cause adverse effects other than stopping the heart (think of a surgical electro knife, or the "electric chair"), but given voltages in the two digit range and currents in the single-digit milliampere range, this simply isn't going to happen (but even if it was, it would primarily be local burns, not life threatening).

Answer 4

Different areas of the world differ slightly, but the 'safe' level for low voltage tends to be pitched at the 40 to 60v range, below which it is deemed that the likelyhood of electrocution is negligible. 'High voltage' cars use 42v, 3 '12v' batteries in series.

120v is sufficiently hazardous that plenty of people die of 120v mains shocks. Apparently more Americans die of 120v than Europeans die of 240v, perhaps due to a sense of complacency over the lower voltage (somebody check on snopes and debunk if necessary!)

Answer 5

First off, some people seem to have fixated on POTS which is nothing like PoE. Props to all the telecom engineers who've had ringing current up the arm on a sweaty day running jumpers.

Further to the good answers above about the safety of 48vDC in itself, PoE is a negotiated protocol - the connected devices start from the fail-safe assumption that the other device is NOT PoE compatible and therefore would not enjoy having power shoved up its interface. Only when two PoE-compatible devices are connected does one see the special signalling that tells it it might want to offer PoE - from there, the two devices will negotiate on what one needs / what the other can deliver and will effectively "work up" towards the required power level. It's also worth noting the power they arrive at could be much lower than the full amount supported - very few devices are that power hungry.

A disconnected or dangling cable (one end plugged into a PoE port) will NOT have an appreciable voltage on it - in fact no more than a standard ethernet cable.

In short, unless your kid manages to chew through a connected & working PoE cable AND bridge the conductors but miraculously WITHOUT causing the devices to spot the fault / lose connection, it's unlikely you could even get to the delicious volts inside before it's all shut down.

WARNING: This assumes real standards-compliant PoE devices rather than the many proprietary ways companies find of getting power up an ethernet cable, which often consists of a wall-wart wired to the spare wires in an ethernet adapter. In that case, you are at the mercy of the power supply - it could be some small anemic low-voltage wall-wart that's no issue at all or it could be a relatively high-current 48v or higher unit which MIGHT be a bit more risky, but frankly unless you've bought some ridiculous non-standards compliant not-really-PoE ethernet-connected floodlight or something you're at low risk.

One example I've seen of relatively high-power PoE-ish devices are high-end CCTV cameras with high-power LED's and full PTZ movement, they can draw 100-200W, but in the home it'd be very unlikely you'd ever find something like that.

Well, you might in my home, but, y'know...

Most other not-quite-PoE devices I've seen are just squirting 12-24v up the spare pairs from a <50w power brick or wall wart.

Answer 6

In Europe, the maximum voltage for child toys is 24 V DC only. If applied directly over the heart at chest and back, even smaller voltages may be dangerous.

Answer 7

IEC 62368-1:2014 defines "safe" as 60VDC or 2mA DC, whichever is less. For AC the limits are 30VAC and 0.5mA. These are considered safe for an ordinary person but you still cannot leave exposed connectors/wires lying around as there's the whole chapter of "electrical fires" to consider.

There are higher limits for an "instructed person" e.g. service techies. These are 120VDC and 50VAC, with a special exception given for 90VAC telephone network ringing. These are defined as being noticeable or even unpleasant but usually would not produce harmful electrical physiological effects.. Current limits are 25mA DC and 5mA AC.

Anything above that is considered dangerous with "normal" frequencies, SMPS circuits might exceed the 100kHz AC limit but that's beyond the scope here.

Answer 8

A couple of other answers mention 48V DC being a "safe" voltage, however I would argue that the environmental conditions (grounding, skin conductance etc) are far more important factors than just voltage.

The following is anecdotal admittedly, but I have experienced a very nasty DC shock working on a 48V DC gyro on a ship after coming inside with wet salty hands.

See also the following stackexchange answer that claims even 24V can be dangerous under certain conditions. How much voltage is "dangerous"?

Answer 9

What is more important than voltage in electrical safety is current. The amount of current required to start or stop the heart is 60 mA across the chest. Under the right circumstances even 12 volts can be lethal. In a marine environment- i.e capsized boat in salt water any person in the current path could be killed. 48 Volts could give you a healthy jolt if your skin is moist with sweat. Under normal circumstances with dry skin 48 Volts would probably wake you up but not kill you. When the the electrical current is strong enough to burn away the layer of skin and fat and comes in contact with the tissue directly below this layer it is extremely conductive then even relative low voltages can be lethal.

AC is far more deadly than DC- One of the reasons it is the preferred for performing an execution in the old days.