0
\$\begingroup\$

So I was watching a recent video for TED-ED titled "How to practice effectively...for just about anything - Annie Bosler and Don Greene," and at one point the speaker said the following:

Myelin is similar to insulation on electrical cables. It prevents energy loss from electrical signals that the brain uses...

I thought cables were insulated just to protect us from getting electrocuted or to protect us from leakage current... Can someone please explain?

\$\endgroup\$
6
  • \$\begingroup\$ If you have two wires going to that lamp on the table over there, no insulation, and the wires are touching, how bright do you think the lamp will be? \$\endgroup\$
    – Trevor_G
    Jan 4 '18 at 3:00
  • 5
    \$\begingroup\$ TED often trades vigor for rigor. So, take TED with a pinch of salt. To quote Nassim Taleb "[TED is a] monstrosity that turns scientists and thinkers into low-level entertainers, like circus performers." \$\endgroup\$ Jan 4 '18 at 4:14
  • 1
    \$\begingroup\$ TED combines motivation, facts, logic and research to summarize a new way of understanding human behavior. Use the "pinch of salt" to stimulate your appetite for learning but don't just rely on this. \$\endgroup\$ Jan 4 '18 at 4:36
  • \$\begingroup\$ consider faint signals instead of power \$\endgroup\$
    – Jasen
    Jan 4 '18 at 5:09
  • \$\begingroup\$ @Trevor The bulb wouldn't light because no current will flow through it due to the short circuit that was formed, right? \$\endgroup\$ Jan 4 '18 at 13:00
4
\$\begingroup\$

Insulation goes beyond just preventing a nasty shock. It prevents bare wires from catching on fire in an un-fused circuit. For wires that have 20,000 volts AC or more a combination of neoprene or silicon insulation with an outer insulator of Teflon helps prevent corona discharge of the energy into the air.

Over distance of many meters coronal discharge can waste away a significant amount of energy. This can also produce a irritating odor known as ozone, which is deadly in high concentrations.

As an example at my previous job we charged a capacitor bank with a 32 kilo VDC source, but it had a high AC ripple in it. The wires were rated for 40 kV but we heard the hiss of coronal discharge leaking out from the 4 meter long pair of wires.

This prevented us from fully charging to the expected 32 kV. So I had to buy and install Teflon tubing 31 mils thick and just slightly larger diameter than the wires. It solved the corona discharge problem.

Insulation goes beyond preventing a shock or a short-circuit. It can prevent high voltage from leaking into the air, which prevents loss of energy, and a major if not fatal shock hazard, and the release of ozone gas.

EDIT: You cannot prevent coronal discharge in high-tension wires. The power company simply accepts the losses, especially in rainy conditions. You might be thinking of the ceramic 'bells' that are used to prevent arcing by creating a series of air-gaps. The high-tension wires hang from these stack of bells, mounted to a steel tower. I have not heard of a 'Coronal Ring' before.

\$\endgroup\$
2
  • \$\begingroup\$ How does the insulator prevent the corona discharge? I only read about corona in high-voltage transmission lines and the only way (that I know of) to prevent it was by using a Corona Ring. \$\endgroup\$ Jan 4 '18 at 13:06
  • 1
    \$\begingroup\$ I added an 'EDIT' section to my answer to cover other circumstances related to coronal discharge. \$\endgroup\$
    – user105652
    Jan 4 '18 at 15:04
3
\$\begingroup\$

The body's nervous system has no metallic conductors for electrical impulses, but instead has electro-chemical salt conduits called axons.

Some axons which are wrapped with Myelin a fatty insulation with a low dielectric constant like oil or plastic ( < 4) and very low electrical leakage ( high resistance) which means it can carry signals (action potentials) far (>1m) and fast (~100m/s) .

Unlike water ( plasma, blood) which has a dielectric constant of ~80, a Myelin coated axon has a very low capacitance which we would call high characteristic impedance, high leakage resistance and high wave velocity compared to say axons used for pain or temperature sensors which do not have Myelin coating and are very slow (~1m/s.)

Myelin coated Axons do make electro-chemical signals faster and more efficient by their electrochemical insulation properties compared to plasma or salt water.

So in a sense it is like plastic coated wires which if submersed in the ocean would protect wire from the high dielectric constant of water and leakage resistance from salt.

ref: https://www.nature.com/scitable/topicpage/myelin-a-specialized-membrane-for-cell-communication-14367205

\$\endgroup\$
2
\$\begingroup\$

The expression "prevents energy loss" is not the best description of the function of electrical insulation. It may be better as a description of the function of myelin. The comparison is is still generally valid. Without insulation electrical signals being transmitted using cables would not get to the destination or would be severely diminished at the destination.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks for the added note about wires that carry only signals. \$\endgroup\$
    – user105652
    Jan 4 '18 at 3:31
  • \$\begingroup\$ Can you provide me with a website link to the importance of cable insulation for electrical signals specifically? After reading your last sentence, I felt like I'm missing something on this topic. I tried to google it, but its no use... \$\endgroup\$ Jan 4 '18 at 16:35
  • \$\begingroup\$ I doubt there is any web site that specifically discusses insulation failure in signal cables. Obviously, if the insulation fails and two conductors touch there can be a short circuit such that the voltage is zero at the receiver. With power distribution voltage levels, the conductors are likely to weld themselves together making a complete and permanent short circuit or make a complete and permanent open circuit by destroying the wire and/or causing disconnect by protective device. A signal cable fault could have a high resistance shorting connection that would diminish the signal level. \$\endgroup\$ Jan 4 '18 at 17:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.