>Dr Alastair Lyndon, from Heriot-Watt University, said: “Underwater cables emit an electromagnetic field. When it’s at a strength of 500 microTeslas and above, which is about five percent of the strength of a fridge door magnet, the crabs seem to be attracted to it and just sit still.
For reference, this is the magnitude of Earth's magnetic field.
What's the strength of magnetic fields around undersea cables?
From the paper:
>EMF strengths predicted around subsea power cables, as reported in the literature, vary from 140–8000 µT [15,17,18]. A commonly utilised cable operating at 1600 A is expected to produce an EMF of 3200 µT in a perfect wire, at the cable surface [17]. As with all EMF, the values will decrease with distance from the source, resulting in a field strength of 320 µT and 110 µT at 1 m and 4 m respectively [17].
426 pages sheesh. Page 25 (37 in the pdf) discusses the magnetic field strength model. No math but references. I'm getting more nervous because the references are all related to modeling power lines in air.
Table B-1 shows a list of submarine power lines and Table B-2 contains their simulated magnetic field strengths.
Medium does affect magnetic field geometry. It'd be nice if someone took some actual measurements if they're not going to use a decent model.
My understanding is that, in an ideal case, the medium only affects epsilon and mu, the permittivity and permeability, which are easily found and substituted from lookup tables.
The lookup tables were made based off of power lines in air.
So much effort has been spent studying the effects but presumably no one has actually checked the magnitude of the magnetic fields around submarine power lines.
I was not referring to the lookup tables in the appendix, rather known lookup tables for relative permittivity and permeability in salt water at 60Hz. If you know those relative parameters you can simply adjust the air-based calculation by multiplying by some function of the ratio of the water parameter to the air parameter (effectively cancelling out).
I'm no expert though, only have some high level undergraduate fields and waves course work.
I'm pretty sure that's due to saltwater's conductivity, not any EMF considerations. Conductivity will result in some partial cancellation of the AC magnetic field. But not enough to matter at 60 Hz, which is within the Extremely Low Frequency band used for communications with submarines.
I just don't understand why you think this matters. Even if the relative permeability were as high as, say, 2, it would only move the 500 uT effect radius by some tens of centimetres. But just from common sense, it must be several orders of magnitude less than that, because otherwise your compass wouldn't work near the ocean. In practice, you need a very sensitive instrument to even measure the relative permeability of water, which already means that it's completely negligible for the purposes of this discussion. Wikipedia quotes water's permeability as 0.999992.
By that measure, an eighty-times stronger argument against the researchers' due diligence would be "how do they know the cable isn't actually carrying 1599 amps? Did they measure the current?"
I think there's a difference between them being "attracted" to the
short range field, and being disoriented as they climb over the cable.
Maybe it's the heat or inductive heating effect they like. EM
shielding the cables would make them too heavy. Switching to DC
transmission might fix it, but the cable loss would be higher and the
cost of rectifying and remodulating to AC would be high.
Two thoughts come to mind.
More cables in parallel carrying less current, which gives the added
advantage of redundancy.
Short breaks in the flow, like a railway crossing for crabs, for 5
minutes or however it takes them to get unstuck and go along their
crabby business.
>Switching to DC transmission might fix it, but the cable loss would be higher
AC loses due to reactive impedance and skin effect should always be higher than DC transmission. DC might make sense if the generation is solar based, as it naturally generates DC, although converting it to higher voltages must have an AC step.
Informative but entertaining video on high power DC transmission from Electroboom. To my recollection it’s generally always better transmission wise except the DC-AC and AC-DC converters are both expensive and physically large.
Both size and cost are dropping pretty quick with the development of better MOSFETs - the same ones that are soon to get very cheap because they're used in electric car motor inverters. I predict that in 100 years the majority of big power links will be DC (although we'll probably still have an AC grid too)
DC is also used because you avoid grid synchronization issues, and this is generally the trend for long distance connects now in general. It means the grids can have frequency divergence without affecting the transmission line itself.
The issue with DC transmission in subsea cables is that the XLPE insulation can become polarized and - from what I remember - have water drawn into it due to the consistency of the field.
You're talking about flipping the polarity, going occasionally from conductor-positive to conductor-negative (the shield must remain at ground potential.)
If I remember correctly that actually makes things much worse if you've operated at one potential for a considerable amount of time.
You could operate the cable at AC ~ 0.1Hz but your ability to actually transmit MWH would suck.
And DC avoids problems caused by induced currents due to geomagnetic storms (you get a voltage shift, but that's no big deal, compared to what happens to transformers.)
grid synchronization 'issues' are a feature, not a bug - by very, very slightly varying how hard to you try to 'push' out of synch you can manipulate power flows at will. This is dramatically more difficult with DC. That, and ease of voltage level manipulation were the reasons so much transmission was AC in the first place!
No they're issues: if it was easy, then no one would bother with HVDC - HVDC exists because it reduces losses and vastly eases grid coordination because conversion to and from AC at either end is handled under local control and can be totally independent of the source/destination.
Its the difference between just shifting the frequency or phase on your local control clock at the substation, and trying to coordinate with another state/territory/nation's entire AC generation infrastructure to keep everything in phase.
Adding more of something is often the simplest and cheapest way to fix problems. Example: Adding a few more VMs or containers costs almost nothing. Paying developers to eke out 10% more performance is expensive.
Another example: Microcontrollers that have loads of pins (e.g. 100) and support many different peripherals (i.e. protocols like I2C) are expensive (say, $10-20). Little ones with a medium amount of pins (e.g. 56) and support for limitless protocols are cheap ($1). So if your PCB needs just a few more GPIO pins to accomplish something it makes more economic sense to just "add another" little MCU rather than "go big". Even if it makes programming the thing slightly more complicated.
Or more abstractions. Where the original problems that we were trying to get away from, resurface in new ways, and previously solved problems have to be reimplemented.
DC-to-AC losses used to be high (historically) because of inefficient inverter technology. These days we have 90-95% efficiency inverters (that are built to match their HVDC voltages).
It's interesting because the reasons why we built everything using AC in the past don't really apply anymore (well, most of the time). DC is more efficient for most domestic and industrial electricity utilization. Things like lighting, air conditioning/heating, refrigerators, etc are all more efficient if fed from DC. Think about it: How many "wall warts" have we got in our homes these days? All our LED light bulbs have built-in inverters and that decreases their efficiency. Our PCs use DC for literally everything internally. The list goes on and on.
I had also read a while back that grid-scale DC power transmission could be more resilient because the power transformers aren't as expensive. It's like $8 million for one of those giant high voltage AC transformers they have at power stations whereas with high voltage DC the equipment is cheaper but you have more of it (or something like that; I wish I could remember the details but this was an explanation given to me from a DC fanboy lineman over 10 years ago). The idea being: If bad weather breaks your giant $8 million AC transformer that's a real expensive pain in the ass but if it breaks a handful of small DC transformer components that's a quick (and cheap) fix.
Actually not really...
A single-phase-AC-sourced rectifier does need a considerable output capacitor, though, and it will have a poor power factor with severe harmonics unless it spends extra on PFC which would not be necessary with a local DC grid supply (say, 320V DC to replace 230V AC systems to match peak voltage; yes the higher RMS voltage results in (anti-)proportionally reduced current, allowing cheaper/thinner wires or less loss).
Many LED bulbs are a simple diode rectifier circuit, essentially with a bunch of LEDs stacked to add up to a total 120V bias voltage. These will blink at 60Hz or 120Hz depending of if they implement a half-wave or full-wave.
More complicated bulbs use an active rectifier / switching power supply instead.
Those are kinda shit, though.
Also, the ones you're thinking of should be capacitive dropper circuits, if I'm not mistaken.
There's also the electrolytic-smoothed bridge rectifier variant.
BigClive has a lot of teardowns of such light bulbs (and non-bulb-shaped alternatives) with circuit diagrams and explanations. They're really good for learning about the various "cheap LED bulb" designs.
Sorry, but no. An inverter uses switch-mode technology to enable voltage conversion. Usually this allows using small high-frequency transformers, rather than large 50/60Hz iron cored units. Which is why so many wall-warts use Inverters to step the voltage down.
Actually, they're much better than "just" 95%.
98-99.5% is realistic with at least non-isolated converters for each end.
And yeah, a DC grid needs active converters at connections points anyways, and the small voltage changes needed for steering power flow across redundant paths is also relatively cheap.
A big issue is just that a constant-power SMPS with an input voltage range/tolerance inherently offers a negative differential (aka incremental) resistance: within the supported input voltage range, reducing the input voltage causes the current consumption to increase inverse-proportionally (typically even a tiny bit worse because internal losses in the SMPS increase with current, requiring even more current to make up for the slight drop in efficiency (think 98% to 97%, for a sense of scale)).
In AC grids phase is comparatively easy to stabilize, and constant-power loads already need PFC that (for single-phase rectifiers) has to emulate a normal resistor who's resistance slowly drifts to accommodate changes in output power consumption. The only real issue is that this method requires an intermediate stage with large capacitors to feed the output stage while the input stage stops feeding near the zero crossing (the capacitors get smaller if you do away with the zero crossing (DC grid using the emulated resistance for stabilizing the grid), but the stages remain, along with their ~1% inefficiency per stage).
The main issue with LEDs is that you need electrolytic capacitors to buffer the zero crossing of single-phase AC, and electrolytic capacitors dry out/corrode when operated at high temperatures.
So unless you want the LED PSU to have a fan, those capacitors will severely affect how tightly integrated you can build a non-low-power LED light bulb.
And yeah, especially at a local scale DC grids are very useful, because you can skip expensive inverters and conversion losses for both battery(-backup) sources and solar panels/fuel cells.
You would typically still want an MPPT for a string of solar cells, but that's a compact and rather inexpensive DC/DC converter. Batteries only need a way to accurately limit the charge voltage/current per-pack, to prevent the voltage drop along the grid cables from affecting the maximum charge voltage/state of the pack's cells. Also a deep-discharge protection, but beyond those, a (mostly) solar-fed local DC grid can just let the MPPTs feed as much as they can up to a target voltage, from where they start to gradually throttle output current to only produce as much as is being used (once the batteries are full). If the batteries start to reach sufficiently low charge state, load shedding or alternative power has to be used (say, via telling PCs to reduce/disable Turbo Boost and similar, or lights/screens to limit brightness).
Probably only historical ones. In the past it wasn't easy to get high voltages with DC, so losses where higher. These days we are better at changing voltages, and so DC losses the same as AC with similar voltages (skin effects still can be a small difference).
I understand we can actually run DC at higher voltages than AC, though I'm not enough of an EE to explain why. IF this is true, then DC losses would be less than AC.
Looks like the underwater power cables are almost "a drug" for crabs, very curious:
>Dr Alastair Lyndon, from Heriot-Watt University, said: “Underwater cables emit an electromagnetic field. When it’s at a strength of 500 microTeslas and above, which is about five percent of the strength of a fridge door magnet, the crabs seem to be attracted to it and just sit still.
>“That’s not a problem in itself. But if they’re not moving they’re not foraging for food or seeking a mate.
>“The change in activity levels also leads to changes in sugar metabolism - they store more sugar and produce less lactate, just like humans.”
Alternative hypothesis: It seems to me that they're being drawn to these power cables like moths to a flame.
There's a comment above in which some guy observes that the strength of the EM field emitted by these cables is similar in strength to the Earth's EM field.
The article also mentions that these crabs migrate. Presumably they migrate using some organ that is sensitive to the earth's magnetic pull.
We know what happens when moths--who use light to navigate--encounter artificial lights. Their navigation hardware can't handle it, and they end up circling the light. This seems like the same kind of thing.
I thought the same thing - reminded me of birds. (I was casually aware that we knew they had some sort of magnetism sensing, but hadn't caught up with the fact that we think it is a visual thing)
The description made it sound like they discovered how to live like a vegetable in front of cable TV. Perhaps this is the final stage of crustaceanification?
Underwater power cables can be designed to have zero electrical and zero magnetic fields emitted from them. Typically that's done by using a grounding shield (which eliminates electrical fields), and by making sure currents in all directions are balanced (eliminating magnetic fields).
As far as I'm aware, nearly all modern high capacity cables are built this way already.
It's only old stuff (which tries to use the earth as a conductor) or low capacity stuff (which is sometimes AC and not well balanced) where this isn't true.
Using a twisted pair cancels out both the magnetic and electric field. But it also more than doubles the cost and the size of the cable. Which is why all existing DC transmission lines use earth return circuits.
Right. Not an electrical engineer, but isn't there something like a
counter-twisted pair (quad) that minimises EMR? Maybe that's tuned
to work best for 50Hz-60Hz?
> Ants are strongly attracted to both AC and DC electric fields
(Fig. 2). The numbers that accumulated increased with an
increase in the voltage for both DC and AC voltages, and for
all species. Some species (P. comanche, Ph. hyatti) show
little or no response to voltages lower than 50 volts. Other
species showed slight responses to lower voltages, although
the response greatly increased at voltages above 60 volts. No
difference in ants' responses was detected between AC and DC
generated fields (Fig. 2), although ants leave AC powered
points at a slower rate due to a possible residual effect of
AC (MacKay et al., 1989). Ants of all three subfamilies
responded in a similar manner.
Volt is not a unit of an electric field. Talking about '50 volts' is complete nonsense in a context of an electric field. Also, a magnetic field around a wire will establish if there is a moving charge inside the wire, which can be established with any voltage -- as long as there is current, which is measured in ampere. Electric charge is measured in coulomb and magnetic fields are given in tesla -- so there cannot be a connection if they talk about 'volt'.
I am not saying there isn't any link but I am saying that 'volt' is surely the wrong connection, so this should be taken with utmost caution.
The unit of electric field strength is V/m. They put ants inside a test apparatus, so the "per meter" part is fixed (by its design) and the voltage is experimentally manipulated. They do use the right units elsewhere in the paper.
I imagine this was done to avoid measuring the electric field in the electrode/air/floor/ant medium, which is not exactly homogeneous.
It's a cheeky reference to power grid cables on land having (negative) influence on the health of humans; and that being dismissed as "conspiracy theory."
Regardless, at a more practical level, sea creatures can't wear hats, tinfoil or otherwise.
> It's a cheeky reference to power grid cables on land having (negative) influence on the health of humans; and that being dismissed as "conspiracy theory."
The thing is, there is no scientifically validated evidence of that effect; also, there is no proof of humans being able to sense (electro-)magnetic fields.
Correct, at least for whatever was measured in the those studies (i.e., perhaps not everything was measured and analyzed). And the parent comment didn't refernce humans.
That said, this report / study does show evidence of electro magnetic influence on living organisms. So perhaps it's worth asking "Have we missed something in humans?" That's a legit question, especially in the context of science.
The obvious solution to this is to change the design of the cable so that it won't emit significant electromagnetic radiation. For example, a coaxial cable will successfully contain magnetic fields for both AC and DC lines.
Yet another reason why the world needs room temperature superconductors. Actually, in this application the water temperature only goes up to 20°C (at the surface) but at the bottom of the ocean in these areas it's more like 10°C. So the bar is slightly lower.
> “This could mean we have a build-up of male crabs in the south of Scotland, and a paucity of them in the northeast and islands, where they are incredibly important for fishermen’s livelihoods and local economies."
Thankfully that prediction can be verified easily before we jump to solutions such as burying underwater power cables.
Which is great, as such a falsifiable prediction is a welcome part of a scientific paper.
Is there any way, even in theory, we could practically carry power without all these wires? Copper isn't exactly eco friendly or all that cheap or common(Which doesn't stop anything from using it in purely mechanical things that could have been plastic or steel....).
> Copper isn't exactly eco friendly or all that cheap or common(Which doesn't stop anything from using it in purely mechanical things that could have been plastic or steel....).
TFA doesn't mention copper at all, the high-tension wires running above my street are composed of stranded tubes of aluminum.
Isn't this the animal that covers Christmas island with population numbers in the billions and we watch cars drive over thousands of them at a time every couple years?
Aww man I feel terrible for the crabs. But I doubt the impact to them, no, I doubt the save-the-crabs-lobby will out influence the under-water-power lobby. RIP brown crabs.
Right right, they produce static magnetic fields, which are rather boring compared to AC’s changing magnetic fields which can induce currents. I would imagine inducing currents could be a problem in seawater.
This is highly, highly dubious without a physical explanation. Or with representative statistical data. Magnetic and electric fields do not interact much with biological matter, so something 'special' needs to be shown here before there is a point.
This should be treated with utmost caution without statistical and physical proof.
This is categorically false, both magnetic and electric senses in other animals have been proven to exist, pigeons for magnetism and the electric organ in bony fishes.
It's not at all physically implausible that crabs sense some electric or magnetic character in the surrounding water through their antennae or some other organ. Electroreception is known in bees, why not crabs? This could easily be the first evidence of that.
But I am not saying that the phenomenon does not exist. I am saying that I am lacking evidence in the article. Evidence could be statistical (if we don't know how yet) or physical (if we do know how, like, a sense was identified). But for me, pointing to a bunch of crabs on a cable and making assumptions is not enough.
No, you were saying it's inherently dubious without either statistical or physical evidence.
I'm replying, no, there's nothing dubious about it. Which doesn't mean it's accurate. Biology has always prominently featured observation from life, and this is, in general, how the field first starts to suspect that, say, bees can sense electric fields.
Experiments on crabs might and should follow, but your skepticism isn't grounded in anything here.
> No, you were saying it's inherently dubious without either statistical or physical evidence.
Yes, exactly, because at least counting the crabs and comparing the count with other clusters of crabs would show whether there 'is something'. Missing accuracy is not my problem, it's missing 'there is something' vs. 'there is nothing'. Without any statistical data, there is nothing to talk about, not in biology either, which is also science.
Skepticism is very important. I challenge them to count crabs on cables and compare with crab counts not on cables, and then make a case. That's all.
That's not right: electric and magnetic fields are used to alter brain activity, even non-invasively.
Transcranial magnetic stimulation uses pulsed magnetic fields produced by a strong electromagnet. It's currently an FDA-approved treatment for depression and a pretty common research tool. The fact that it has (some) biological effect is pretty clear: aim it at motor cortex and you can make someone's finger (leg, etc) twitch!
Transcranial electrical stimulation (tES) is a family of techniques that apply weak electric current to the scalp to affect brain activity. These include tDCS (with Direct Current) and tACS (Alternating Current). While their effectiveness has been debated, my group has collected data demonstrating robust effects on the activity of neurons, even deep in the brain. Check out Figure 1 of this paper: https://www.pnas.org/doi/10.1073/pnas.1815958116 It's not always that simple, and it interacts in complicated ways with ongoing brain activity, but it does do something too.
You can get much strong effects if the electrodes are inserted directly into the brain (or muscle) too but it's not all the case that EMFs are biologically "inert".
This is crackpottery. What is the mechanism of action that such a weak low frequency AC magnetic field could effect the biology? There is none. This makes about as much sense as that crazy neighbor who lives without power and rants about how they can feel cell phones.
Neurons and muscles are electrically active, so an external electric field can weakly polarize a cell, and thereby alter its response to ongoing activity.
They're not magnetically active, but moving conductors in a magnetic field produce current, so it's not wildly implausible. Indeed, if you were a crab getting unexpectedly stimulated, sitting still would be one way to make it stop!
The rate of change in current, dI/dt, determines the rate of change in magnetic field and so induced voltage. In order to depolarize neurons in transcranial magnetic stimulation dI/dT in the magnetic coils placed directly above the tissue is about 2000-6000 amps per millisecond. Even a lot of AC power at low frequency in a single wire pair (or whatever) is not in any circumstances going to induce a voltage across or along a membrane or even in extracellular space that can effect the polarization of a neuron.
So it is clear that changes in the magnetic field inducing voltages cannot be the mechanism of action in this case.
TMS produces suprathreshold stimulation: it can take a neuron from its (passive) resting potential to spike threshold, all by itself. The numbers you give are correct for that.
Weaker stimulation can't do that, but subthreshold stimulation can "nudge" the timing of spikes. In a living brain, neurons are constantly bombarded with synaptic input, and therefore sit closer to spike threshold and occasionally spike. Weak stimulation can therefore cause them to fire a bit sooner or later than they otherwise would. Since information is carried not just by the overall amount of firing, but also the spatiotemporal structure of that activity, these subthreshold changes can lead to overt changes in behavior. The recurrent nature of the brain can also amplify these effects.
I know this because I've literally measured it in macaques and found that fields well below 1 V/m (produced by ±2 mA 5-40 Hz AC on the scalp) are enough to reorganize spike trains. Here's one of our papers: http://packlab.mcgill.ca/Krause%20et%20al%202019.pdf
FWIW, I was surprised by these results--and deeply relieved when they replicated!
How can you conflate your direct electrode transcranial electrical stimulation paper with non-contact magnetic field rate of change induced electrical fields? That's just unsupportable. You're not going to get anywhere near 1V/m across the brain tissues with even a kiloamp of low frequency (ie, <500 Hz) AC current non-contact. Rate of change matters for induced fields.
Cool paper and impressive but the direct contact, part of the circuit, tES results don't translate to this context. The induced voltages (for the power cable) would be so far below the noise floor that not even arguments for stochastic resonance boosting above threshold make sense.
For reference, this is the magnitude of Earth's magnetic field.
What's the strength of magnetic fields around undersea cables?
https://www.mdpi.com/2077-1312/9/7/776/htm#B15-jmse-09-00776
From the paper: >EMF strengths predicted around subsea power cables, as reported in the literature, vary from 140–8000 µT [15,17,18]. A commonly utilised cable operating at 1600 A is expected to produce an EMF of 3200 µT in a perfect wire, at the cable surface [17]. As with all EMF, the values will decrease with distance from the source, resulting in a field strength of 320 µT and 110 µT at 1 m and 4 m respectively [17].
15: https://dspace.lib.cranfield.ac.uk/bitstream/handle/1826/778...
426 pages sheesh. Page 25 (37 in the pdf) discusses the magnetic field strength model. No math but references. I'm getting more nervous because the references are all related to modeling power lines in air.
Table B-1 shows a list of submarine power lines and Table B-2 contains their simulated magnetic field strengths.
Medium does affect magnetic field geometry. It'd be nice if someone took some actual measurements if they're not going to use a decent model.