Hi,

This is my first post here and just have a question about the use of a 40 cal suit. Here is the situation I found myself in.

Most of my work deals with solar. I was working on a 100 amp 500v DC combiner box on a roof. We were not the installing contractor but doing a service call to the site. I previously did a site survey here and found this to be one of the most dangerous and sloppy work I have ever seen. Normally my 12 cal pants, 6 cal shirt and 8 cal jacket with hard hat and arc shield would be enough to protect me. But knowing how dangerous the installation I wore my 40 cal suit with full hood as I just felt more comfortable knowing I had the extra protection.

Well low and behold this is what my safety director said to me after I turned in my JSA for this service call. Her words "Also, perhaps I misunderstood you but, the Blast suit should only be worn momentarily when turning on switches/equipment. It is not intended for FT use". Is this correct that it should not be worn all the time?


thanks
steve

Hello,
Welcome to the forum.
You make the statement that "knowing how dangerous the installation I wore my 40 cal suit". Besides the comment on the previous contractor, which I assume to be poor workmanship, what made you decide that your normal PPE was not enough? What is the site labeled for incident energy? While poor workmanship may increase the risk of an arc flash incident, it will not increase the severity.

I would say that the 40 cal or any PPE, needs to worn for as long as the hazard exists.

What was the tA?

I just did calculations for a 2 MW solar inverter that has a 1000V bus, @256A per leg (total Bus I = 4096A)
the Cal/cm2 for was only 3.52 with a tA of .03 / 17.74 cal/cm2 with a tA of .1 (those values were at the recommended x3)

I did calculations for work at the inverter and used .003 tA because of the DC Fuses we use

Larry...have you yet found ANY DC system over even 4 cal? I have not so I'm pretty much convinced the 70E DC tables are nowhere near reality.

I have not, but in all our DC systems we use fuses with very short interrupt ratings. I could not get the cal/cm2 of the DC portion of the 2MW solar inverter over 4 cal/cm2 without pushing the tA up to 10 times what the fuse interrupt rating is.

In my case the typical systems that I see are DC 120 V power supplies for substations frequently including battery arrays, the DC buses inside of drives and UPS's, electrostatic precipitators which have extremely high voltages but almost no current, DC motors on various track mounted mining equipment (trolley motors), and DC generators which are low voltage, high current. I just haven't found anything yet except that I have heard of one case of an extremely large UPS but that's about it.

Follow the company policy. Overkill doesn't really help you that much. Hot sweaty clothing is less protective due to the sweat. Paul is right, the DC tables are overkill but they were developed that way to be safe. Do calculations and wear PPE to match the calcs. LV arcs don't have much plasma that can reach anything other than the gloves. Preventing ignition is the key and 4 cal/cm2 does that perfectly.

When you have a 40 cal/cm2 potential event, wear the suit. We are learning that something is almost always better than nothing.

I work for a Variable Speed control manufacturer. Most DC drives are 500V armature. We do make some 575V and 690V drives as well. On AC drives the DC bus is typically around 700V. We make up to 2000 Hp drives.

On the Grid Tie Inverters we are working with 500 - 1000 VDC with up to 16 inputs rated up to 256A per leg. We make up to 2MW Grid Tie Inverters

Still, the greatest danger is not the DC bus but the tie in on the isolation transformer to the grid.