External Consumer Unit

[Cbelectrics]

Good morning guys

Was looking for advice with regards to installing an outdoor consumer unit. Customer is looking for power to be installed from there property to an area in there garden. There installing a future hot tub and summer house and any future lighting and power. They currently have a small log cabin where I was going to situate a 4 or 6 way consumer unit which can feed any future points. My question is would it be best to fit a standard 18th edition metallic board inside a ip Rated cabinet? I've seen online there are ip65 consumer units which are plastic but obviously that wouldn't meet current regs. I'm just wondering if there is any other things to take into consideration with regards to temperatures

Any advice would be appreciated

Would advice the cabinet if outside just as good practice. While I can’t remember any specific regs for that one it would still come under making sure what you are installing is appropriate for the environment it is in.

Not sure what system you are on but you would also need to check your Ze readings to make sure the earth extension (from the supply) doesn’t make it go above the required level of 0.35 or 0.80. If it does it would need its own earth rod as it may not comply with the required trip times on the new ccu breakers.

 

[mainline]

Not sure what system you are on but you would also need to check your Ze readings to make sure the earth extension (from the supply) doesn’t make it go above the required level of 0.35 or 0.80. If it does it would need its own earth rod as it may not comply with the required trip times on the new ccu breakers.

I think you're getting a bit confused Between Ze and Zs.

 

[Cbelectrics]

I think you're getting a bit confused Between Ze and Zs.

No i am not. If you increase your Ze by extending your main earth conductor then you increase your Zs (Zs= Ze + (R1+R2)readings and therefore your circuit trip times (max Zs readings allowed in the regs so it disconnects in the appropriate time)That is why you have a max limit of ze in the regs of .35 and .80.

 

[Cbelectrics]

It is also why extending a DNO’s earthing arrangement is frowned upon for that very reason.

In most situations it won’t make much of a difference. However if someone does the work without that knowledge and installs a cable that’s too small that Ze reading will increase. Also if you live further from the DNO’s step down transformer the Ze will also increase before you have even thought about extending the earthing arrangement. So should also check a previous EICR for the Ze reading at the main head.

 

[westward10]

No i am not. If you increase your Ze by extending your main earth conductor then you increase your Zs (Zs= Ze + (R1+R2)readings and therefore your circuit trip times (max Zs readings allowed in the regs so it disconnects in the appropriate time)That is why you have a max limit of ze in the regs of .35 and .80.

You only have Ze at the source of supply everything else regarding the earth fault loop path is Zs. 0.35 and 0.80 are applicable to Ze neither of which are quoted in BS7671.

 

[mainline]

No i am not. If you increase your Ze by extending your main earth conductor then you increase your Zs (Zs= Ze + (R1+R2)readings and therefore your circuit trip times (max Zs readings allowed in the regs so it disconnects in the appropriate time)That is why you have a max limit of ze in the regs of .35 and .80.

The O/p Isn't extending his Main DB into the shed

 

[pc1966]

As above, Ze is at the supply point of the whole installation, and Zs (or Zdb if you prefer) at other locations. There is nothing special about the 0.35 / 0.8 ohm values, they are typically met from domestic TN-C-S or TN-S supplies but not guaranteed.

However, in the TN-C-S case they relate very closely to the DNO side supply drop under load, so if too large you will be failing on the ESQCR voltage range regulations anyway.

Beyond that your check on the loop impedance should be to establish if it is a fault or not, and if too high for your planned supply fault protection then you need to look again at it, perhaps a fuse for sub-main instead of a MCCB/MCB, or go down to route of a delay RCD for fault protection even on a TN supply.

 

[davesparks]

No i am not. If you increase your Ze by extending your main earth conductor then you increase your Zs (Zs= Ze + (R1+R2)readings and therefore your circuit trip times (max Zs readings allowed in the regs so it disconnects in the appropriate time)That is why you have a max limit of ze in the regs of .35 and .80.

Ze is the earth fault loop impedance of the external supply, that what the e stands for, external.

The Ze will stay the same regardless of what you install or where.

The regs do not set any limits for Ze. DNOs generally try to keep within the 0.35 or 0.80 values, but supplies can go above these limits. Generally when designing new distribution DNOs work to lower limits, I think UKPN have a design stage limit of 0.25 for a TNCS supply.

 

[Cbelectrics]

Fair enough on the point of Ze but it still applies that your measurements will increase at the new db effecting trip times.

 

[westward10]

Fair enough on the point of Ze but it still applies that your measurements will increase at the new db effecting trip times.

Zs will increase with Ze being a constant.

 

[davesparks]

Fair enough on the point of Ze but it still applies that your measurements will increase at the new db effecting trip times.

Yes measurements will increase, but that does not mean there is any problem, if everything is designed correctly the increased values will still be compliant.

 

[mainline]

Also if you live further from the DNO’s step down transformer the Ze will also increase before you have even thought about extending the earthing arrangement. So should also check a previous EICR for the Ze reading at the main head.

Personally, I would take a Ze measurement myself rather than rely on someone else's maybe drive by scrap of paper.