TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

[Jim Specs]

TN-C-S System Max Zs for Final Circuit Protected By 30mA Type B 16A RCBO

I have read a few articles stating that under BS7671, max permissible Zs earth loop impedance for above circuit is 1667 Ohms.

1667 ohm permissible Zs for above circuit protected by 30mA RCD would only seem to satisfy BS7671 clause 411.4.4 in regards to fault current. There is also a requirement to provide overcurrent protection i.e. MCB/RCBO. Table 41.3 details max Zs for the overcurrent characteristics of RCBO which is 1.37 Ohms for Type B 16A RCBO. A 1667 ohm Zs would seem to be non-compliant in regards to overcurrent characteristics of RCBO detailed in Table 41.3

Granted at best, Zs can only infer the impedance of line/neutral for overcurrent short circuit scenario . But never the less, this is the requirement of Table 41.3 for the overcurrent characteristics of an RCBO.

Interested in your thoughts on this.

 

[James]

If you have protection against shock sorted by the rcd component, why are you concerned about over current when it will be sorted by the Mcb part of the device and cares nothing about the earth loop.

 

[Jim Specs]

If you have protection against shock sorted by the rcd component, why are you concerned about over current when it will be sorted by the Mcb part of the device and cares nothing about the earth loop.

I am concerned with BS7671 non-compliance.

In a scenario where we have an overcurrent short circuit between line/neutral, it is my understanding that the RCD integrated into the RCBO will not detect the short circuit and will not disconnect. The MCB integrated into the RCBO should detect the overcurrent short circuit and disconnect. However if the line/neutral impedance is excessively high, for example 1667 Ohms, then it will take a longer time before it will disconnect. I would think this could lead to serious issues.

I am curious to know why people do not adhere to Table 41.3 max Zs for the overcurrent characteristics of an RCBO.

 

[westward10]

Your line/neutral will not be that high.

 

[Jim Specs]

Your line/neutral will not be that high.

Agreed, it is very unlikely.

Line/Neutral impedance needs to be much lower to ensure that the RCBO can provide effective overcurrent protection. Perhaps a suitable Line/Neutral impedance would be something similar to the Zs values detailed for overcurrent characteristics of RCBO in Table 41.3

 

[westward10]

The protective device will not differentiate between a line/earth fault and a line/neutral fault it just reacts to excessive current regardless of how it is derived.

 

[Tom256]

You are confusing the need to meet ADS (Zs value) with the need to calculate using the Adiabatic equation for overcurrent (granted only really of value at disconnect less than 5 seconds and ideally <0.1 seconds).

Consider thermal effects. The RCD does not provide protection against the impacts of overcurrent because it is not an overcurrent protective device. Overcurrent impacts include the thermal effects as a result of fault current or overload current. It is not only shock that needs to be considered.
The RCD as the name refers is for residual current.

There are no energy let through values for rcd's because the standards work on the basis that the rcd will be backed up with an overcurrent protective device. Even with RCBO's the values for let through and make / break relate to the mcb aspect of the device.
In your example, a TN-C-S with Zs of 1667 ohms, well we can stop at that point because at those values you would have a serious issue regardless and that needs sorting out. You would never design a circuit on those values, how could you even do so... But lets go with someone did and the values improved at some future point now what ?

 

[Tom256]

However if the line/neutral impedance is excessively high, for example 1667 Ohms, then it will take a longer time before it will disconnect. I would think this could lead to serious issues.

How long do you think it would take for your 16 A rcbo to disconnect using the figures in your example and what would be your concern?

 

[Jim Specs]

You are confusing the need to meet ADS (Zs value) with the need to calculate using the Adiabatic equation for overcurrent (granted only really of value at disconnect less than 5 seconds and ideally <0.1 seconds).

Consider thermal effects. The RCD does not provide protection against the impacts of overcurrent because it is not an overcurrent protective device. Overcurrent impacts include the thermal effects as a result of fault current or overload current. It is not only shock that needs to be considered.
The RCD as the name refers is for residual current.

There are no energy let through values for rcd's because the standards work on the basis that the rcd will be backed up with an overcurrent protective device. Even with RCBO's the values for let through and make / break relate to the mcb aspect of the device.
In your example, a TN-C-S with Zs of 1667 ohms, well we can stop at that point because at those values you would have a serious issue regardless and that needs sorting out. You would never design a circuit on those values, how could you even do so... But lets go with someone did and the values improved at some future point now what ?

Yes, I am aware that an RCD does not provide overcurrent protection, as I have stated in my original post above an MCB/RCBO has to be provided for overcurrent protection.

Agreed, a consultant should avoid producing a design that has very high impedances.

My post concerns BS7671 Max permissible Zs for Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System.

Max permissible Zs for above circuit is ?

 

[Tom256]

Yes, I am aware that an RCD does not provide overcurrent protection, as I have stated in my original post above an MCB/RCBO has to be provided for overcurrent protection.

Agreed, a consultant should avoid producing a design that has very high impedances.

My post concerns BS7671 Max permissible Zs for Final Circuit Protected By 30mA Type B 16A RCBO on a TN-C-S System.

Max permissible Zs for above circuit is ?

How have you designed the circuit? Is the 30mA RCD for Additional Protection? What is the Volt Drop?

 

[Tom256]

I am concerned with BS7671 non-compliance

Further to my last: What is the non-compliance?

 

[IAmSparkytus!]

This post feels very “A.I.” to me

 

[Tom256]

This post feels very “A.I.” to me

Yea.

 

[Tom256]

Table 41.3 details max Zs for the overcurrent characteristics of RCBO which is 1.37 Ohms

Table 41.3 details max Zs times as specified (not typing it all out) .
If you are looking at overcurrent there is more to be considered. 1.37 ohms is not the requirement to be met for selection of suitable overcurrent protection between live conductors.

Even with Zs values that are high (relative) on a TT system for fault current and cpc sizing the worst case must be considered and that can be many times higher than that calculated by the RA value.

 

[Jim Specs]

The protective device will not differentiate between a line/earth fault and a line/neutral fault it just reacts to excessive current regardless of how it is derived.

In regards to my original post, it's not quite that simple.

For the circuit described in my original post, the RCBO will typically disconnect in a shorter time duration for line/earth (fault current) due to the RCD integrated into the RCBO. As the RCD only requires a relatively small amount of current to trip this leads to a higher permissible Zs for BS7671 Clause 411.4.4.

When determining max permissible Zs for circuit in my original post, I am also considering Table 41.3 for the overcurrent characteristics of an RCBO.

There is a distinct difference between line/earth fault current and a line/neutral overcurrent that needs to be considered in the context of my original post.