help with college formula please!

[hoppy]

Just been doing some college work.

Was calculating if supp equip bonding was required in a bathroom. The formula given for an mcb is R < 50V / Ia.

Ia being the fault current for a particular mcb to disconnect in a certain time.

My question is, as there are likely to be various circuits in a bathroom, which mcb rating would you use as the calculation. Would it be the shower circuit mcb as this is likely to be the highest rated and thus require more fault current to disconnect within a certain time.

I know to the 17th, that supp bonding is not required if rcd protected and all that. This is a theory question and was just wondering what you guys thought.

Cheers
Craig

 

[ezzzekiel]

Supp bonding is used when there is an increased risk of electric (ie bathrooms) and the resistence between parts should be less than 0.05 ohms.

another use is when disconnection times cannot be met, so if the circuit you are concerned with will not disco in the required time then supp bonds are installed using the formula you provided and the fault current taken for that device.

Althouh now under 17th most requirements are met to avoid installing sup bonds at all (although personally I will always do it regardless.

Thats my view/ understanding im pretty sure youll get other opinions.

 

[electricAl]

That pretty much covers it. Working to 17th now you`d have an 30mA RCD protecting all ccts in the location so you could supplant the Ia for I^n to give you R< 50v x 0.03

Understand your Q was a theoretical one but in the real world the 0.05 ohms is the biggie - resistance between all exposed & extraneous conductive parts, & between those & the MET to be that or less.

So, if calculating for Supplementary Bonding required due to increased shock risk, then establishing no simultaneously accessible parts have greater than 50mOhm resistance between them is what is required

Whereas, when Supplementary Bonding so disconnection times are met then this is required for each cct in the location

 

[xraytek]

Just been doing some college work.

Was calculating if supp equip bonding was required in a bathroom .

In theory if no RCD then yes, supplementary bonding is required regardless of any calculations (701.415.2). But then 701.411.3.3 says RCD's are required regardless

The formula given for an mcb is R < 50V / Ia.
Ia being the fault current for a particular mcb to disconnect in a certain time.

This formula is to confirm if supplementary bonding is effective, not to see if its required or not.
R is the resistance between accessible exposed conductive parts and extraneous conductive parts (415.2.2).

Would it be the shower circuit mcb as this is likely to be the highest rated

The higher the value of Ia the lower the value for R, so its fair to assume that provided the shower complies than the others will if their measurement is not a greater value.