TB said:

, is there any recomendations on arc rated ppe that would be good to carry, or if in doubt, it wouldn't be a bad start to wear this?

Some contractors we work for issue Skanwear STRATA overalls to electricians as standard workwear making a basic level of protection almost automatic.

Since we tend to have an idea of the types of equipment and conditions we’re likely to encounter, we’ve been able to run the calculations on likely scenarios. As a result, I have an overalls that come out whenever access to live switch rooms is anticipated plus helmet, gloves etc to bring up the protection level that travels with me and an awareness of where that protection is insufficient. Being standard issue means these pre-assessed scenarios can also be factored into the RAMS; slightly easier for us being consultants as we can mitigate the risk by limiting access and witnessing from further away but of course “space suits” can be obtained if really needed.

You can probably do similar to at least cover your initial approach and survey of equipment, and then carry out a job-specific assessment from there.

There problem with unknown systems is unknown fault currents and duration ! That may sound trite, but it is key. There is a huge difference between the maximum flash you can possibly face downstream of a 13A fuse, vs a 100fuse, vs a 400A slow blow one.

(but you can estimate - assume I2t * half the supply voltage is the energy available to create the bang, the other half of the supply voltage going in the supply internal impedance, and imagine that energy spread out over a sphere with the fault at centre - as the outer surface area of the sphere is 4.pi.r^2 you can quickly get to joules per square cm. About 5 of those on bare skin is enough for an instant painful sun-tan and much more gets you into into hospital trips and by about ten times that on large areas of the body means effects you don't recover from.)This sets a safe distance of closest approach for bare skin.

There are also non-flash arc effects, like being basted in droplets of molten metal, that can make the scope for injury from accidents like spanner slipping between the phases suddenly far greater. Meanwhile rated light coloured cotton overalls, and certainly long sleeved tops of the right materials are so cheap that not having that would almost be reckless and already make a huge difference by reducing how much body is exposed.

Having the mentality of deciding at the outset, this is just a goggles and gloves job, or this a  visor and gauntlets job or this is a use a pointy stick job and doing that grading as a dynamic process may help.  PPE is something you need to have the habit of selecting at the outset, not 'oh I'll go and get it if  I need it."

The trick then maybe to get good at looking at something and deciding what the level of risk is.  Simply put the problem is one of total fault energy, as above, times a focussing factor that depends on the enclosure - you may get energy reflected off the back and sides of a box in a way that makes having your body near the open front  2 or 3 more dangerous than if the fault was dangling in free space - the effect is that the enclosure protects the  things behind and to the sides from the exposure, but does that at the expense, at least partly, by redirecting that blast of energy that would have gone that way forwards instead.

M.

Note if its  not clear that when comparing the labels on things  like those overalls may say

"EN 61482-2:2020 ATPV = 12cal/cm² " 

This is a test of a sample of the fabric, not the overall or whatever.

The ATPV of the fabric is the energy density on the outside of the cloth, that skin on the other side would receive the upper limit 1.2calories (5 joules) dose through it. If course if folk have clothing beneath, that attenuates the effects further and makes things safer but is very hard to control . Poor underwear choices, like synthetic melt and stick fabric can make things a lot worse very quickly.

Some fabric manufacturer’s report an EBT or Energy Break Through value for the garment. EBT is the point at which the fabric burns through. Clearly the ATPV is more use and the EBT is quite a bit higher.

M.

Thanks for the replys. sounding like I've got the right idea at least in planning ahead and trying to learn as mich as possibel. Its just a very deep and winding rabbit hole that I'm in!

Now just neet to get to grips with some of the finer points in estimating the likely hazards etc.

My thinking on PPE at the minute is to have some Arc rated trousers and polo/tee shirt for day to day working with a boilersuit that I can put on over the top if the hazards are higher.

Has anyone any experience with gloves such as these: https://gsworkwear.com/products/portwest-arcgrip-gloves-a780?variant=15698900156459 I was thinking somthing like this for day to day working but wondered if they are really offering any worthwhile protecation?Then with gauntles if needed.

Plus the helmet, ear defenders, glasses, virsor as and when needed.

Out of interest, how much of an arc flash risk exisitis on an average domestic/light commercial installation when carrying out things like external earth loop impedance tests? I figured this is probably the highest risk as its at the cutout/main DB, live and with some exposed live parts. Obviously it varies with the exact installations but for something around 2kA prospective fault current with the standard DNO 80amp fuses for example, would you be looking at a a full gaultlets and visor set up to carry out that test, or would glasses, basic gloves and long sleves be reasonable protection.

Thanks

I've got Skanwer's technical team doe to phone me later in the week to have a chat with them, I'll see what thye've got to say as well. Hopefully it'll be a worth while phone call and not just a sales call.

A crude worked example !

80A BS1361 fuse perhaps the ferraz shawmmut BME42V80

shows a total energy to open is 30,000 Amp² seconds.
We have to have a figure for the Zs of the supply, lets take a fairly bosky supply capable of a 230V open circuit and 20kA PSSC, so for the worst case, the energy into the fault and drop on the supply is equal, and 10,000A flows and 115V is dropped creating approx 1Megawatt of energy, but for how long before that fuse pops.
so the time is 30,000 divided by 10000A squared,(which is 100,000,000 A ²⁾ so the equivalent energy rectangular pulse duration would be ~ 1/3000 second (but course as the fuse 'blows' the current is not constant, but then neither is the switch off time a true knife edge). So worst case arc energy is the same as that megawatt for 1/3000 second , so about 3000 joules. Now if that was in free space, we would have to spread that over at least 600 square cm to get down to the "safe" (burnt bare skin, not that safe) limit, and that is actually not that large a sphere, about half a metre diameter or 25 cm ~ 10 inch radius. Redo the calc for 10 times the power density - about the "12 calorie" level for the overalls or thick gloves, divides the dangerous sphere diameter by about 3 (where 3 squared is almost 10) to  less that 10cm/ 4 inches But as above, the enclosure either saves you or could makes it 3 times worse depending where you are in relation to any focusing action.
This sort of wet finger sum only should be used to give a feel for how far back to stand ! There are a lot of approximations, and the 'formal standards grade' formulea are quite a bit more complex and nuanced...
Mike.