Isolating 120VAC

[fraymond]

I am looking to "shield" a bank of 120VAC solid state relays. Currently the SSR's are in an enclosure along with some instrumentation amplifiers. The amps' sit on a shelf and the SSR's are below. My contention is I don't want an unqualified/non-arc flash wearing employee entering into the enclosure unless I can remove the potential from the SSR's (which means he has to be trained, suit up, and check if the potential has been removed). I was thinking an alternative is to barricade/isolate the SSR's by using a piece of phenolic or acrylic. Another thought is to set a non-metallic enclosure with door inside the existing enclosure and move the SSR's inside of it. What are your thoughts, concerns, etc. Thanks

 

[wbdvt]

If the whole cabinet is 120Vac, then it is exempt from arc flash hazard analysis per IEEE 1584-2002 and we label this as <1.2 cal/cm2 so non melting clothing is worn along with appropriate shock protection. There is no need to "suit up".
You can place a piece of clear acrylic plastic over the exposed 120V circuits but you should consider heat. You may be restricting cooling air flow. The other alternative is training the individuals that have to enter that cabinet on the shock hazards of 120Vac. You can then provide them with Class 00 gloves, insulated clamps and a insulating material that they can hang over the relays to prevent inadvertent contact.

 

[fraymond]

Thanks wbdvt for the response. If I were to put the SSR's in an enclosure with proper ventilation wouldn't the shock hazard as well as the need to wear protective clothing be eliminated? Part of my concern is what happens if the employee 'forgets' to wear the proper clothing or gloves. The risk would still exist. However putting the potential in a closed enclosure removes the risk. Please point out my flaws in my logic. Thanks again

 

[wbdvt]

What protective clothing are you referring to? If they are electrical workers they should be in non melting clothing to begin with which is all that is required for the 120V work. Now your facility may have had an arc flash study done and determined that there is a minimum arc rated clothing they should be wearing so there is no clothing issues unless your workers are coming to work naked. hmy:

If the exposed 120V circuits are adequately protected from inadvertent contact, then I would say you have eliminated the hazard. Once the hazard is eliminated, then the need for the associated PPE goes away.

This statement concerns me the most:

Part of my concern is what happens if the employee 'forgets' to wear the proper clothing or gloves.

This implies that either the workforce is not being trained on the hazards and how to protect from them, not taking training seriously, not doing a pre-job brief before the task or there is a lack of following and enforcing the workplace rules. One way to find out is to perform employee observations and document them. There are certain sections of OSHA that require periodic audits of programs/work and this may fall under one of those. Regardless, it is a good practice to observe the employee's work to look for areas that need improvement.

 

[fraymond]

At first I was miffed about your comments but I realized you are right. We recently had a training session for unqualified employees. I need to check the employees training records to ensure he has been trained. Also I should have said non melting clothing. Throughout the year we have monthly triaining in the hazards of arc flash, LOTO, and yearly LOTO assessments on the equipment and operators as well as other safety topics. We also observe and document empoyee risk behavior on a daily basis. Our plantwide arc flash assessment begins this month as well as our switchgear maintenance. We do have a program that I believe may not be as good as some but better than most. You fortunately wbdvt have pointed out a gap that will be addressed. As I mentioned I will isolate and ventilate the SSR's. Thank you for helping to make us better.

 

[Electric-Light]

I am looking to "shield" a bank of 120VAC solid state relays. Currently the SSR's are in an enclosure along with some instrumentation amplifiers. The amps' sit on a shelf and the SSR's are below. My contention is I don't want an unqualified/non-arc flash wearing employee entering into the enclosure unless I can remove the potential from the SSR's (which means he has to be trained, suit up, and check if the potential has been removed). I was thinking an alternative is to barricade/isolate the SSR's by using a piece of phenolic or acrylic. Another thought is to set a non-metallic enclosure with door inside the existing enclosure and move the SSR's inside of it. What are your thoughts, concerns, etc. Thanks

Take a look inside a PC. All dangerous voltage parts are caged inside the power supply. PCs are meant to be user serviceable, so there is no open frame power supply in there. The instrumentation/logic side wires that flip the power supply on and off goes inside the cage, but its isolated to thousands of volts from the utility side optically through a photo couple. You have to pour liquid or poke inside the cage to breach it. It's likely the SSRs you're using is a built this way as well. Could you not pig tail into instrumentation box in another cabinet terminate the control input to SSRs onto a terminal block inside the instrumentation cabinet? It's a good measure anyways. Having instrumentation wires accidentally touch line voltage stuff is a good way to mess up the day anyhow.

 

[fraymond]

Take a look inside a PC. All dangerous voltage parts are caged inside the power supply. PCs are meant to be user serviceable, so there is no open frame power supply in there. The instrumentation/logic side wires that flip the power supply on and off goes inside the cage, but its isolated to thousands of volts from the utility side optically through a photo couple. You have to pour liquid or poke inside the cage to breach it. It's likely the SSRs you're using is a built this way as well. Could you not pig tail into instrumentation box in another cabinet terminate the control input to SSRs onto a terminal block inside the instrumentation cabinet? It's a good measure anyways. Having instrumentation wires accidentally touch line voltage stuff is a good way to mess up the day anyhow.

Thanks for the response Light. Your analogy of a pc is good. Doesn't my proposed acrylic barrier serve the same purpose?