What the curriculum thinks you need to know:
PC_BK_43 Electrical hazards: causes and prevention
PC_BK_44 Electrocution: including microshock, earth faults, leakage
PC_BK_45 Electrical equipment safety: domestic and medical, classification/types of equipment, symbols
What you need to know (The theory):
Electricity causes two things when it comes into contact with the body:
- Activation of nerves/Muscles (think muscle contraction, VF etc)
- Burns (e.g. like in Diathermy)
If there is one sentence to remember when talking about electrical safety and electrocution it is: Current kills.
Now, If we think about this, to get current to flow through the body, we need a few things:
- An electrical source – e.g. a bare live wire, faulty piece of equipment, electrified train rail etc etc
- A route to ground/earth
Take a look at the electrical basics post, current flow is dependent on a few things. Ohm’s Law again…
So, I = V / R.
So to get current to flow we either need a large voltage OR a low resistance (to ground/earth) or, even better, both.
In terms of trying not to get shocked, the voltage is mostly out of our control, unless we design our own equipment that is. So we can’t change it. However we can change the resistance our body provides to current flow.
If you measure the resistance from one hand to another with a multimeter, you typically have a resistance in the region of 100-250k Ohm (try it for yourself).
Now imagine you stuck your finger in a plug (please don’t!), current would want to flow through you to ground. Your body provides resistance to flow, but if there is no other lower resistance route, it will flow through you to ground. Ouch.
Now, Imagine you have just got out of the shower, are wet, and stick your finger in a plug. Your skins resistance is dramatically lower when wet. It drops to 1.5-2.5k ohms, so 100x lower than dry skin.
So, to get the same current flow as in dry hands, as you have 100x lower resistance, you need 100x lower voltage.
From the table above, you can see the currents required to have different effects on the body. If you’ve ever put a 9v battery on your tongue, that is about 9mA of current. This is about the maximum you can get away with.
Now, you’ll notice that its actually quite hard to get enough voltage to get current flow with 100k resistance. However, the skin is a dynamic thing and reacts/degrades to high voltage currents. With 120v AC at 50Hz, the resistance quickly drops to ~1800 Ohms, and this is where you get electrocuted. The amount of time the current is in contact with the skin also determines clinical effect.
From the graph you can see in terms of causing muscle contraction, increasing length of contact has a linear effect on reducing the current required to activate muscles. In terms of VF, increase your time of contact from 0.1sec to 1 sec and you also reduce the current needed to produce VF by 10x.
Okay, so why don’t we get shocks with diathermy?!
That has to do with current frequency. The worst frequency of AC current for electrocution and VF potential is 50hz. Its just annoying its also one of the most efficient to move electricity around a national grid, so that’s why its used.
So if we want to avoid getting shocked, we need to:
- Don’t stick fingers in plug sockets (e.g. make sure live electrical components and wires are insulated and enclosed)
- Make sure there is an alternate, low resistance route to ground/earth (e.g. earth our equipment)
- Increase our resistance to ground/earth (high resistance footwear, don’t have wet hands).
Imagine you put connected a mains cable to a internal pacing wire. The resistance from the to the heart and out of the body is massively decreased due to you bypassing the skin. This puts the patient at massive risk of shock.
Because of this any wire into the body is high risk for causing shock and VF. This is the reason your central line wires have insulated tips.
Insulate the wire, job done. Well not exactly. There is a thing called leakage current. This is where small amounts of current ‘leak’ from an insulated wire. The process which causes this is called capacitive coupling. So it has something to do with capacitors… Have a look at the Capacitors and defibrillators post to get more about this.
A capacitor is two conductive plates separated by an insulator or ‘dielectric’. Imagine your wire going into the patient is one plate and the wires insulation is the insulator/dielectric. Then, the patient makes up the other conductive plate. You can quite easily see how AC current could flow into the patient in the same manner as a capacitor because of this. This is called ‘leakage current’. Each implantable device will have a certain amount of leakage current. The higher the leakage current, the further from the heart you have to keep it.
What you need to know (How it works in practice):
What you need to know in practice is how safe a particular piece of equipment is to use in a particular patient setting. Helpfully, there are a set of symbols that help with this…
Look around your house at electrical equipment and you’ll see the above symbols on the manufacturing labels.
Class I: This is electrical equipment which is grounded/earthed. Meaning the casing (or any user accessible and conductive bits) are connected to earth. The advantage of this is that if the casing should become live, it is connected to earth so you don’t get shocked.
Class II: This is equipment where there is a double layer of casing to double insulate electrical parts from the user. With the advent of plastic equipment, this is now by far and away the most common type currently.
Class III: These are devices with a ‘floating circuit’. This means the circuit has no direct connection with the mains supply. They receive power through an isolating transformer or a battery.
Medical equipment however needs a little more thought. Devices that we connect to a patient, or even worse into a patient can cause serious risks of shock. These need to be rated for the amount of current which they leak:
The above symbols define how much leakage current these devices produce. Unsurprisingly, the ones with the heart involved loose less current and are suitable for connection to a patient’s heart (e.g. a Swan Ganz catheter or similar) than the ones with a body.
Type B: Loose 100uA or less. (note type BF are the same, but with a floating circuit)
Type C: Loose 10uA or less (these all are floating/low voltage & current to enable this low a leakage)
Note symbols in the middle of a capacitor sign are suitable for usage where a defibrillator will be used…
Electrical safety devices
There are many devices which aim to stop you getting shocked. These generally work by shutting off the supply current when the current flowing through the circuit exceeds a predicted point. This excess current flow is usually produced by the current flowing to ground/earth.
Fuses: These are simple pieces of wire in series with the live connection of a circuit. When current flows through the piece of wire it heats up. Depending on the constituent metal and thickness of this wire, if a certain current is exceeded then the fuse will melt, shutting off the supply current. Simple. Except they are quite annoying to replace as anyone with a very old style fuse box in their house will attest.
Circuit breakers: These have generally replaced fuses in ‘fuse boxes’. These work on the principle of an electromagnet. As current flow increases, it charges an electromagnet which pulls the live connection contacts apart, shutting off the current. The problem with this is it also switches off the electromagnet! So, a latching spring setup is used to hold the contacts apart until you reset it manually (which takes much less time than changing a fuse wire in a fuse…).
Random Exam factoids (i.e. the things the college like asking):
- Electrical symbols are a favourite you may not think about in the OSCE examinations. Remember to look over them!
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