spieczonka
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I am looking to see what the requirements are for the installation of a single string battery disconnect on a 24V DC 1200amp disconnect. And also if there is a spacing requirement for it, as there are with AC Boxes.
24V DC power requirements
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Re: 24V DC power requirements
spieczonka, is one of the polarities grounded, and is the disconnect fused??. I have installed several battery disconnects and battery plants on 24 and 48 VDC, but need to know if the system is grounded and/or fused.
My first piece of advice is not to use a disconnect unless local jurisdiction requires it.
spieczonka, is one of the polarities grounded, and is the disconnect fused??. I have installed several battery disconnects and battery plants on 24 and 48 VDC, but need to know if the system is grounded and/or fused.
My first piece of advice is not to use a disconnect unless local jurisdiction requires it.
Re: 24V DC power requirements
Dereckbc, I have a -48V DC system for a Telecom system; the positive side is grounded, and the system has no overcurrent protection devices. You mentioned on your previous post that you should not use a disconnect unless local codes require it. Why is that? Does the NEC require that the battery have a overcurrent protection device? Please site NEC section #. Should I protect both legs of the battery or just one leg, and which one? Can I use a fused disconnect switch? gaglibu
Dereckbc, I have a -48V DC system for a Telecom system; the positive side is grounded, and the system has no overcurrent protection devices. You mentioned on your previous post that you should not use a disconnect unless local codes require it. Why is that? Does the NEC require that the battery have a overcurrent protection device? Please site NEC section #. Should I protect both legs of the battery or just one leg, and which one? Can I use a fused disconnect switch?
gaglibuRe: 24V DC power requirements
You mentioned on your previous post that you should not use a disconnect unless local codes require it. Why is that?
This is a real gray area of the code. If you reference 480.4 you are required to use over current protection but no requirement for a disconnect device. The reason most Telco?s do not like using either device is if you were to open the disconnect device or loose a fuse, the battery goes into self discharge. After a period of time the voltage will drop, say below ? volt, of the system voltage of 53 volts. If that happens, and you were to close the disconnect or fuse you would create a tremendous amount of charge current which can and will litterly weld or pit the contacts. So if you use these devices you should have alarms to indicate the condition, and have a portable rectifier to bring up the battery voltage before connecting back to a live plant.
Does the NEC require that the battery have a overcurrent protection device?
Yes, 480.4. But it is not typically done on the conductors between the batteries and the plant charge buss. Some Telco?s are exempt from NEC requirements, but not all.
Should I protect both legs of the battery or just one leg, and which one?
Either one or both, it depends on the design. If you were not going to ground either polarity you would fuse both polarities and use GFD (Refer to NEC 250.162). But the industry standard for -48 VDC battery plant is to ground the positive charge bus (called the return bus), and install overcurrent protection on all ungrounded circuit conductors on the primary and secondary distribution system. For +24 VDC you ground the negative buss and do the same as the -48 VDC.
It?s similar to AC distribution except you do not size circuit conductors per 310.16, GEC?s per NEC 250.66, and EGC per 250.122. Instead you base the size of circuit conductors on length and voltage drop, and all ground conductors based on DC resistance. Using this method you will always exceed NEC requirements unless very short distances are encountered in which case you would base on NEC requirements (rare).
Just remember you cannot tolerate much voltage drop on 48 (2-volt max), 24 (1-volt max), and 12 (1/2 volt max) systems, Fault clearing paths (frame ground network) have to be very low resistance in order to operate OCPD at low voltages.
[ October 09, 2003, 01:41 PM: Message edited by: dereckbc ]
You mentioned on your previous post that you should not use a disconnect unless local codes require it. Why is that?
This is a real gray area of the code. If you reference 480.4 you are required to use over current protection but no requirement for a disconnect device. The reason most Telco?s do not like using either device is if you were to open the disconnect device or loose a fuse, the battery goes into self discharge. After a period of time the voltage will drop, say below ? volt, of the system voltage of 53 volts. If that happens, and you were to close the disconnect or fuse you would create a tremendous amount of charge current which can and will litterly weld or pit the contacts. So if you use these devices you should have alarms to indicate the condition, and have a portable rectifier to bring up the battery voltage before connecting back to a live plant.
Does the NEC require that the battery have a overcurrent protection device?
Yes, 480.4. But it is not typically done on the conductors between the batteries and the plant charge buss. Some Telco?s are exempt from NEC requirements, but not all.
Should I protect both legs of the battery or just one leg, and which one?
Either one or both, it depends on the design. If you were not going to ground either polarity you would fuse both polarities and use GFD (Refer to NEC 250.162). But the industry standard for -48 VDC battery plant is to ground the positive charge bus (called the return bus), and install overcurrent protection on all ungrounded circuit conductors on the primary and secondary distribution system. For +24 VDC you ground the negative buss and do the same as the -48 VDC.
It?s similar to AC distribution except you do not size circuit conductors per 310.16, GEC?s per NEC 250.66, and EGC per 250.122. Instead you base the size of circuit conductors on length and voltage drop, and all ground conductors based on DC resistance. Using this method you will always exceed NEC requirements unless very short distances are encountered in which case you would base on NEC requirements (rare).
Just remember you cannot tolerate much voltage drop on 48 (2-volt max), 24 (1-volt max), and 12 (1/2 volt max) systems, Fault clearing paths (frame ground network) have to be very low resistance in order to operate OCPD at low voltages.
[ October 09, 2003, 01:41 PM: Message edited by: dereckbc ]
Ultrahighgain
Member
dereckbc said:
Could I please have an NEC reference for this scenario?
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