RCCB selection for 3 x Solarlake 17000TL inverters in horticultural location

[Monchichak]

Hi There

Trying to rectify a solar installation also containing a double socket in a horticultural location.

Current setup does not have any RCD protection.

Solar setup: 3 phase TNC-S supply - 3 x Solarlake 17000TL inverters - 50kW installation

The problem I am having is selecting the correct RCD's.

RCD 1. Main RCD supplying feed to horticultural unit
RCD 2. Option 1. Individual RCD's feeding each inverter in horticultural unit
Option 2. Single RCD feeding 3 inverters in parallal


Unfortunately Solarlake has collapsed but I found contact details of a company offering support who recommended the following: '100mA per phase which means 300 mA per inverter which means 1A per 3 inverter Type A'.
I requested the DC current generated by each inverter to determine if a type A RCD was suitable to protect all 3 inverters in parallel but the technical support would not provide me with a figure & kept repeating a type A device would be suitable. What type would you install or more info needed?

This is what the manual says:
RCMU breaker should be installed between inverter and grid, and its rated fault current need to be 100 mA≤Ifn≤300 mA, 0.1S. No load should not be connected to the inverter directly.
I don't understand what this is supposed to mean: 100 mA≤Ifn≤300 mA?

With such a high Earth leakage would it be better to provide each inverter with it's own independent 300mA RCD rather than designing a system with a combined 1A earth leakage? Unfortunately that adds to the materials cost.

How can I meet Special locations Reg 705.411.1 'In Horticultural circuits, whatever the type of earthing system, the following disconnection devices shall be provided (Option iii) In all other circuits, RCD's with a rated residual operating current not exceeding 300mA' for the RCD supplying the distribution circuit to the barn & option 2 if that is chosen?


Thanks for your advice

 

[Monchichak]

Napit helpline to the rescue:

I was calculating the Earth leakage wrong.

A 3 phase device with 100mA per phase earth leakage requires 100mA RCD - not 300mA RCD as I had thought. Feeling a little embarrassed but hey ho - everyday's a school day.

So the solution is:
300mA source Type A RCD supplying 3 x 100mA Type A RCD's supplying the PV inverters independently. Satisfies selectivity & special location reg.

 

[davesparks]

A 3 phase device with 100mA per phase earth leakage requires 100mA RCD - not 300mA RCD as I had thought. Feeling a little embarrassed but hey ho - everyday's a school day.

A device with 100mA leakage will trip a 100mA RCD

 

[Monchichak]

Right you are thanks

The sum of the leakage currents shall not exceed 30% of the RCD residual current

So there is not way to satisfy Reg 705.411.1 with 3 x inverters connected on the same feed unless a risk assessment can be carried out to prove that an RCD is not necessary - which it can't.

 

[davesparks]

Right you are thanks

The sum of the leakage currents shall not exceed 30% of the RCD residual current

So there is not way to satisfy Reg 705.411.1 with 3 x inverters connected on the same feed unless a risk assessment can be carried out to prove that an RCD is not necessary - which it can't.

So you need to design a solution that provides appropriate protection and record it as a departure if necessary.

 

[Monchichak]

Thanks Dave

Due to the location (clients prized horses running sharing barn with kit) I do not think any exceptions can be made so the solution I came up with is to run another 2 cables so each circuit (inverter) has an independent connection to the supply with 300mA RCD & overcurrent protection.
Sounds easy but on paper it becomes a total mess due to all the enclosures required. Luckily Eton do a combined 3P+N RCD MCB for a reasonable price which allows me to enclose the whole lot in a 3P+N dist board with common incoming isolator.

1 more thing - Surge protection. Since there's quite an investment here I figure surge protection would be recommended which hopefully I can fit in the same board.

What do you think?

 

[davesparks]

Thanks Dave

Due to the location (clients prized horses running sharing barn with kit) I do not think any exceptions can be made so the solution I came up with is to run another 2 cables so each circuit (inverter) has an independent connection to the supply with 300mA RCD & overcurrent protection.
Sounds easy but on paper it becomes a total mess due to all the enclosures required. Luckily Eton do a combined 3P+N RCD MCB for a reasonable price which allows me to enclose the whole lot in a 3P+N dist board with common incoming isolator.

1 more thing - Surge protection. Since there's quite an investment here I figure surge protection would be recommended which hopefully I can fit in the same board.

What do you think?

Earlier you said it's a horticultural installation, but now there's horses?
Which is is agricultural or horticultural?

Are you aware that horses are more sensitive to electric shock than humans so a 30mA RCD is unlikely to protect them, let alone 300mA.

The best way to protect the horses is by physical barriers to prevent any possible contact along with robust wiring systems such as steel conduit.

 

[Monchichak]

Agricultural.

I'm aware livestock are a lot more vulnerable to electric shock.
This is an existing installation poorly installed so I am trying to rectify the errors to make it safe & meet regulations.

The equipment is at full ladder height well out of reach of animals or people.
Since it's Agricultural (thanks) the installation cannot be TN-C so it will need to be changed to TT which requires RCD protection.

One thing I am worried about is one of the inverters earth connections has melted so there must have been a voltage difference at some point or fault with considerable current flowing but there was no RCD to disconnect it. The supply has very bad volt drop & I believe it's a TNC-S earth which may be the culprit, thinking about it, if it was the Neutral under load pulling the Earth to create the potential difference that melted the Earth terminal I don't think an RCD would protect against this either. There is currently no bonding.

Thanks for your input

 

[davesparks]

Agricultural.

I'm aware livestock are a lot more vulnerable to electric shock.
This is an existing installation poorly installed so I am trying to rectify the errors to make it safe & meet regulations.

The equipment is at full ladder height well out of reach of animals or people.
Since it's Agricultural (thanks) the installation cannot be TN-C so it will need to be changed to TT which requires RCD protection.

One thing I am worried about is one of the inverters earth connections has melted so there must have been a voltage difference at some point or fault with considerable current flowing but there was no RCD to disconnect it. The supply has very bad volt drop & I believe it's a TNC-S earth which may be the culprit, thinking about it, if it was the Neutral under load pulling the Earth to create the potential difference that melted the Earth terminal I don't think an RCD would protect against this either. There is currently no bonding.

Thanks for your input

You're contradicting yourself now, you said earlier that due to the horses the risk assessment says you can't deviate from the regs. Now you say the equipment is completely out of the reach of the animals and people.

If there's a volt drop problem then you've git other problems, solar installations have much tighter volt drop limits than normal installations.

 

[Monchichak]

I'm only responsible for the solar installation but I have been in discussion to have the volt drop rectified. The inverters have been throwing up lots of errors & going offline due to their strict operating parameters.

Even if the equipment is out of reach it still has potential to cause fault currents great enough to create a fire/shock hazard. I know 300mA will not protect against direct contact but it will reduce hazards a lot more than no RCD.

You have made me think that perhaps I could omit the supply RCD & fit a combined RCD MCB for each inverter in the barn. This would make things a lot easier, save on material cost & means I won't need to run 2 more sub-mains out which sounds crazy but I'm trying to do things correct regardless of cost or inconvenience. All circuits would be RCD protected & meet the special locations reg. The only thing that would not be RCD protected is the SWA supply cable which I can produce a risk assessment for.

Can I ask - how would you do it?

25mm SWA supply is fed from fused isolator within a unit attached to a large estate house, runs underground 6 meters to the corner of a barn, up through the barn 4 meters to a 3 phase 4 way distribution board which supplies 3 x inverters & 1 socket via relevant isolators etc. Current non IP Rated distribution board has been exposed to the elements so may require replacement anyway.