100mA delayed RCD tripping before the 30mA at DB. TT System

[DaveyD]

Hello

Just not on the job at the moment but the builder called. He plugged his small angle grinder in to a new circuit and it took out the RCD at the supplying DB not the 30mA at local CU.

Here's the set up:


TT . 100A DNO fuse

House CU old and full so split tails into small CU with 100A/100mA time delay RCD feeding a CB . SWA to outbuilding/studio and duel RCD CU.

upstream RCD 100mA Timed x1 at 310ms

Both downstream RCD's 30mA checked as perusual and passed on all seven tests.

Electrode at house and new electrode at outbuilding/studio (New at 1.87 Ohms).

My question is ... why has the time delay rcd tripped and not the 30mA? Thank you in advance for your time.

Regards, Dave

 

[Engineer54]

They are 4ft each, I stated there were two coupled together - thats more than a meter <2.5m. What are you suggesting E54? I'm not offended by the way I just don't get where you are going with this? lol

Sorry my mistake, and pleased to hear that you have installed 2X Rods at both locations

You stated across 2 posts that you had 2 electrodes one for the house and another for the studio. and 2 X 3/8'' coupled. Well i know 3/8'' rods can't be coupled together so i've been taking it they were 2 X single rods...

 

[DaveyD]

Sorry my mistake, and pleased to hear that you have installed 2X Rods at both locations

You stated across 2 posts that you had 2 electrodes one for the house and another for the studio. and 2 X 3/8'' coupled. Well i know 3/8'' rods can't be coupled together so i've been taking it they were 2 X single rods...

OK, thanks. I was talking to an engineer (retired) tonight and he suggested that I might try isolating the earth electrodes from each other. I have them joined at both ends at the moment - found this IET Forums - Outbuilding Supply from House on TT System

 

[JD6400]

We had this about a year ago on one of our clients farms , they also run a advents company there as well and erect marquess for it on the old tennis courts .
The supply has a MEM 100a , 0.1 amp s type at the head and a MEM 100a 00.3 at the marquee board . but the 0.1 s type would go first when a certain fridge was used .
After a bit of head scratching my other director who was dealing with this one decided to phone MEM technical , and i believe they said that if the the stake on the head had a significantly lower Ra than the one at the marquee then it would be enough to take out the s type first , he knocked a few more rods in to get it down and it has since been OK .
If i remember to ask him tomorrow for the exact readings involved i will post them up !

 

[DaveyD]

We had this about a year ago on one of our clients farms , they also run a advents company there as well and erect marquess for it on the old tennis courts .
The supply has a MEM 100a , 0.1 amp s type at the head and a MEM 100a 00.3 at the marquee board . but the 0.1 s type would go first when a certain fridge was used .
After a bit of head scratching my other director who was dealing with this one decided to phone MEM technical , and i believe they said that if the the stake on the head had a significantly lower Ra than the one at the marquee then it would be enough to take out the s type first , he knocked a few more rods in to get it down and it has since been OK .
If i remember to ask him tomorrow the exact readings involved i will post them up !

That's so helpfull of you DP - thank you I do appreciate it. That would be interesting. Regards, David

 

[Engineer54]

We had this about a year ago on one of our clients farms , they also run a advents company there as well and erect marquess for it on the old tennis courts .
The supply has a MEM 100a , 0.1 amp s type at the head and a MEM 100a 00.3 at the marquee board . but the 0.1 s type would go first when a certain fridge was used .
After a bit of head scratching my other director who was dealing with this one decided to phone MEM technical , and i believe they said that if the the stake on the head had a significantly lower Ra than the one at the marquee then it would be enough to take out the s type first , he knocked a few more rods in to get it down and it has since been OK .
If i remember to ask him tomorrow for the exact readings involved i will post them up !

They would need as you say, be significantly lower/different...

 

[IzzyS]

OK, thanks. I was talking to an engineer (retired) tonight and he suggested that I might try isolating the earth electrodes from each other. I have them joined at both ends at the moment - found this IET Forums - Outbuilding Supply from House on TT System

Surely they can't be significantly different at the moment can they?

 

[JD6400]

I have spoken to him this morning and they were Hagar not MEM and it was a string of lights that had been wired with a dead short live - earth fault in the middle of the run .

Now E54 please don't shoot the messenger !:grinningelf:
But the reading on the the incomer rod was 0.2 ohms and the one at the tennis courts was 16 ohms .

I know it sounds like they are impossible readings but to give i little context , when i say farm what he actually owns is a small hamlet built around the stock yards and grain stores with his ( And i am being honest here , if you pm me i will forward a link off forum to the farm website ) small castle in the middle of this lot , which the whole lot in turn is sited on a medieval village earth works !

It don't stop there ! The whole site was until around 20 or so years ago a ministry of defence listening post which was peppered with 100-150ft aerials , the large highly secure 1 - 2 acre compound which they fed back to is still there but privately owned by him , with the tennis courts situated about half way between ( 150 m or so ) the incomer to the farm and the compound .
The incomer to the farm is about 5m away from the hamlets transformer with the compound on its own MASSIVE transformer , and the whole lot is on a private network because of it's past ties to the ministry ! I know this for sure because there was a fault in the feed from the next village going to the compound last year caused by Central Networks , so they had to supply a temp genny , but as it was a private network we had to make the connections for them !?:38:

Sorry for the ramble but i hope that gives a better picture as to the readings .

 

[Chr!s]

Hello

Just not on the job at the moment but the builder called. He plugged his small angle grinder in to a new circuit and it took out the RCD at the supplying DB not the 30mA at local CU.

Here's the set up:


TT . 100A DNO fuse

House CU old and full so split tails into small CU with 100A/100mA time delay RCD feeding a CB . SWA to outbuilding/studio and duel RCD CU.

upstream RCD 100mA Timed x1 at 310ms

Both downstream RCD's 30mA checked as perusual and passed on all seven tests.

Electrode at house and new electrode at outbuilding/studio (New at 1.87 Ohms).

My question is ... why has the time delay rcd tripped and not the 30mA? Thank you in advance for your time.

Regards, Dave

A few Questions Dave

1) does the 100mA TD feed the old and new or a separate 100 mA TD for new and old?

2) Are the Earthing systems of the old and new connected or separate from each other?

 

[Engineer54]

Now E54 please don't shoot the messenger !:grinningelf:
But the reading on the the incomer rod was 0.2 ohms and the one at the tennis courts was 16 ohms .

No need to shoot the messenger, ...lol!! That head electrode could well be driven to a substantial depth if this was an Ex MOD property with a substantial ''listening station''. Could well be linked to other earth electrodes (earth field/mat) too....

 

[DaveyD]

Hi Chris, thanks for your interest .....

A few Questions Dave

1) does the 100mA TD feed the old and new or a separate 100 mA TD for new and old?

They are separate. Head>Meter>Tails>henley> SPLIT
one pair of tails go to old house CU which incorporates its own 100mA delayed RCD.
Another separate pair of tails go to new enclosure containing my upstream 100mA delayed RCD and a CB. > SWA> Studio split load CU.

2) Are the Earthing systems of the old and new connected or separate from each other?

The earthing is connected to each other ie I have NOT isolated the house electrode from the studio electrode

 

[DaveyD]

I have spoken to him this morning and they were Hagar not MEM and it was a string of lights that had been wired with a dead short live - earth fault in the middle of the run .
<<SNIP>>.

Thanks anyway for getting back tto me DP.

 

[Chr!s]

Hi Chris, thanks for your interest .....


They are separate. Head>Meter>Tails>henley> SPLIT
one pair of tails go to old house CU which incorporates its own 100mA delayed RCD.
Another separate pair of tails go to new enclosure containing my upstream 100mA delayed RCD and a CB. > SWA> Studio split load CU.
The earthing is connected to each other ie I have NOT isolated the house electrode from the studio electrode

I don't see how two differing electrode resistances could effect the operation of an RCD. The source will just see a level of fault current.

The first RCD will see the leakage of the distribution and the studio.
The second RCD will see the Studio only.

It maybe that the 30mA is being saturated with some DC fault current where as the 100mA is no so susceptible.

Have you checked to see the level of leakage in normal use?

 

[DaveyD]

Thanks for your reply Chris. Not been back yet. I don't have an earth leakage tester but will ramp the rcd's with and without loads to get an ideas of earth leakage.

 

[KFH]

I had a similar situation but with only one earth point at the incomer. Turning on a fluorescent light in the garage, supplied by 50m of SWA with a T to a barn, tripped the 100mA S type, I had just fitted, at the incomer and not the 30mA in the garage. Stripped everything down looking for earth leakage. Replaced 100mA front end RCD and no problems. Both RCDs had tested OK with good times and ramp tests.

Manufacturer would not believe that !00mA RCD was faulty. I sent it to them and they sent two new ones back. Assumption being that the faulty RCD was overly sensitive to transients/interference. The HF fluorescent I had fitted in garage then failed with a burnt out board which may have been causing the transients/interference.

 

[DaveyD]

UPDATE: OK, rechecked everything. That 30mA RCD passed all seven tests again today. Ramped at 21mA
The supplying 100mA TD ramped at 70mA.

Odd.