Minimum Clearance of live parts 1

[tulum]

I have just started doing work in the US and Canada and was wondering....

In Canada CSA regulations for Switchgear Assemblies have a lower minimum spacings in air for live parts than what I see from the NEC Table 490.24 (see below).

CANADA US
2400v 2.5" P-P, 2" P-G 4.5" P-P, 3" P-G
7200v 4.0" P-P, 3" P-G 5.5" P-P, 4" P-G
13800v 6.0" P-P, 5" P-G 7.5" P-P, 5" P-G

What I want to know is "technically" who's right?? ...and don't say it depends on what country your in!

 

[stevenal]

Sounds like you are speaking of manufactured assemblies. NEC does not apply.

 

[jraef]

OK, I won't say it.

However, that is the issue you will ultimately need to deal with, because "technically" it doesn't matter. Under the right circumstances, you could mount the components 1/8" apart and have them be just fine. However, that would be in the vacuum of space, and even then a little space dust may become problematic. The standards adopted by each authority are reflective of the conditions they deem tolerable and / or reasonable to expect. These are not "technical" absolutes, but practical ones since they will be inspected and approved by the Authority having Jurisdiction and violating THEIR rules is rarely up for discussion!

Quando Omni Flunkus Moritati

 

[tulum]

Thanks to both of you. It is appreciated.

I have 2 questions with regards to your responses:

stevenal: Why do manufactured assemblies have special rules..(sounds like a dumb question, but air is air...)?

jraef: I am not quite up on my Latin (just guessing that it is Latin??)... but what does "Quando Omni Flunkus Moritati" mean??

Thanks again for your help...

 

[stevenal]

Different committees, goals, politics, different spacings. Sorry, best I can do. Anyone else?

The wires on my table lamp are much smaller than the circuit I plug it into also. Isn't copper copper?

 

[buzzp]

Spacing of manufactured assemblies is more controllable than spacing concerns of installations. (NEC = installations; CSA/UL=manufacturers).

 

[alehman]

It's interesting that the NEC specifies spacing but not leakage length. Also, the BIL rating of equipment is in part determined by the spacing. Higher BIL requires more space. Spacing can be reduced if the bus is insulated.

 

[jraef]

Manufactured equipment has guidlines to follow such as ANSI, NEMA, IEC etc., but ultimately it must pass stringent testing for certification, i.e. the BIL testing mentioned above, short circuit bracing etc. etc. The test standards are harder to meet than what an inspector can see at a field installation, so they (the inspectors) rely more on "over engineering" by way of the code in their jurisdiction when determining spacings, clearances etc. Its that over engineering issue that is the variable from place to place. Manufactured equipment is closer to your "technically" correct standard, but test standards differe from country to country as well. This is a constant source of headache for manufacturers too.

"When all else fails, play dead"

Quando Omni Flunkus Moritati

 

[jghrist]

Installation standards such as the NEC have different purposes than manufacturing standards such as ANSI.

The NEC is a safety standard and is concerned with clearance of people to live parts. The clearance of live parts to ground or phase-to-phase is usually not a safety concern if the parts are not accessible to people. Equipment is manufactured to ANSI standards which is concerned with the ability of the equipment to withstand voltages, either power frequency or surges.

 

[jbartos]

Suggestion: A distinction should be made among:
1. Creepage distance
2. Creep distance
3. Clearances
3.a. Fence safety, IEEE std 1119-1988 withdrawn
3.b. The minimum separation between two conductors, IEEE 524-1992
3.c. The clear distance between two objects measured surface to surface, ANSI C.2-1997 NESC
etc.
Reference: IEEE Std 100-2000 Dictionary

 

[tulum]

Thanks everyone..great responses.

Stevenal,

"The wires on my table lamp are much smaller than the circuit I plug it into also. Isn't copper copper?"

What am I missing??

The wires on your table lamp were sized with enough copper for an Amp or 2... the wires in your house have enough copper for 15 amps.... copper is copper...

 

[jbartos]

Comment: The design basis documents including criteria for the US and Canadian clearances would reveal details.
It makes a difference if voltage peaks (and how high) are considered and imbedded in the clearance values.
NFPA 70-1999 Art. 490-24 "Minimum Space Separation" states "These values shall not apply to interior portions or exterior terminals of equipment designed, manufactured, and tested in accordance with accepted national standards."

 

[stevenal]

Just an example of the different standards. House wiring per NEC is sized for the overcurrent protection, lamp wiring is sized for the load per NEMA. Maybe a water heater on a dedicated circuit is a better example. Wires neck down at the water heater connection, even though all see the same load and are protected by the same breaker.

 

[tulum]

Stevenal,

Gotcha...thanks