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 Post subject: Handling 120v control cabinets
PostPosted: Mon Jan 24, 2022 8:52 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 30
I was asked why we don't put arc flash labels on control cabinets with a single 120v circuit feeding them.

My answer was essentially, we use the incident energy method on all equipment where the 1584 model applies. Since this falls outside of this, we could use the table method if we can confirm the parameters apply. If the cabinet has only one 120v circuit present in the cabinet exposed, arc flash protection is not needed according to the table. if there is more than one phase exposed in the cabinet, arc flash protection is needed and as long as we can confirm the parameters apply, our basic uniform at 8 cal is adequate. The problem is that we dont know if the parameters apply until we essentially perform a IE analysis with our software.

How do you handle questions like this? Should I add anything else? Is there a model available to determine IE for 2 phase 240v or do you just run it as 3 phase and use the 1584?

Any references to 1584 or 70e would be helpful. I have a feeling i will have to defend this position. Of course i ended with that caveat that i am not an engineer, just an arc flash tech and this is just my opinion.


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 Post subject: Re: Handling 120v control cabinets
PostPosted: Mon Jan 24, 2022 10:14 am 
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Joined: Tue Jul 01, 2014 8:40 am
Posts: 110
Hi and greetings from a very snowy Great North. I have taken Jim's class on arc flash calculations several times over the years including Jim's latest session on the IEEE 1584 2018 edition. To my knowledge I do not believe that there are IEEE 1584 calculation methods for single phase systems such as what you have described 120/240 V. Typical residential or light commercial facilities. I simply tell workers up here including contractors that when working on these voltage systems, to be more concerned with shock hazard as opposed to an arc flash that could potentially cause a 2nd degree burn resulting in the worker being taken to a Burn Unit. Perhaps educating your workers on the extremely low risk of arc flash injury on single phase systems, however there could be shock hazard if the conductors or circuit parts are exposed and that they act accordingly with safety equipment or PPE.


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 Post subject: Re: Handling 120v control cabinets
PostPosted: Mon Jan 24, 2022 10:29 am 
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Joined: Thu Apr 18, 2019 11:42 am
Posts: 85
I believe 1584 has some text that essentially says, "...use a 3 phase system to model a single phase system to yield conservative results." or something like that. In the case of 120V control panels (I'm assuming you mean the panel is fed via a 120V circuit and does not have a 3 phase control power transformer to step down the voltage), then there is no arc flash risk. If you do a search on the internet, you can find a paper called "Arcing Fault Currents in Low Voltage AC Circuits" and "Low Voltage Arc Sustainability". The main gist is that circuits 250V and lower had a maximum arcing current clear time of 10 cycles. This ties in to the PPE table in the NESC.

Mike


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 Post subject: Re: Handling 120v control cabinets
PostPosted: Wed Jan 26, 2022 10:04 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 30
Do you not put any label on these? Our workers all take a 3 hour arc flash training every two years. I'm a little forlorn that they are asking why we don't have arc flash labels on these cabinets.


Leonard wrote:
Hi and greetings from a very snowy Great North. I have taken Jim's class on arc flash calculations several times over the years including Jim's latest session on the IEEE 1584 2018 edition. To my knowledge I do not believe that there are IEEE 1584 calculation methods for single phase systems such as what you have described 120/240 V. Typical residential or light commercial facilities. I simply tell workers up here including contractors that when working on these voltage systems, to be more concerned with shock hazard as opposed to an arc flash that could potentially cause a 2nd degree burn resulting in the worker being taken to a Burn Unit. Perhaps educating your workers on the extremely low risk of arc flash injury on single phase systems, however there could be shock hazard if the conductors or circuit parts are exposed and that they act accordingly with safety equipment or PPE.


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