Overcurrent protection on PV

[paulsamuel1984]

Hi all,

Quick question for everyone. Having had an electrical contractor carry out some of our installs in the busy period, something has arisen that I am unsure about.

For a commerical install we have x amount of inverters feeding a PV DB, then one outgoing cable to feed back into the consumers electrical installation.

Each inverter is protected by an MCB, with a 125A Switch Disconnector at PV Board. If we are feeding back into the existing electrial system we install appropriately rated MCB (and RCD if required) in existing DB.

There have been instances where we have connected directly onto isolator terminals, or split a cable with a junction box. In this instance I can see no evidence of overcurrent protection installed (only RCD). is overcurrent protection required if you integrate with electrical system this way or will existing circuit protection suffice for this?

PS

 

[malcolmsanford]

You could apply reg 433.3.1 (ii) for the omission of overload protection on a PV installation, because your system is never going to carry an overload current, as it is a defined load, ie on a 10kw array, it is never going to reach 12kw

You still though have to provide fault protection for the circuit, and you can not omit a device in environments that are at risk of fire or explosion

 

[paulsamuel1984]

Thanks Malcomsanford,

Trying to justify it, it please correct me if I am wrong: Fault protection is installed as per inverter. If I was to create a new circuit and feed into an existing DB then a new MCB is required to allow for fault protection.

If however, we are tapping into and existing circuit, then this circuit should have existing overcurrent protection which should suffcie for the install, Thus negating the need for additional fault protection.

Is this justification correct?

 

[moggy1968]

you cannot put PV on an existing circuit. It must have it's own dedicated circuit.

 

[paulsamuel1984]

Hi Moggy 1968,

The PV has its on dedicated circuit on 95% of our installs, however there are instances where we have connected directly onto isolator terminals without any overcurrent protection. I.E we have done this to avoid standby generator interference on some sites.

However I have seen one instance where the installer has piggy backed onto an existing breaker - something that I have said is non-compliant.

In your opinion, would you still consider these methods of connection non-compliant?

 

[moggy1968]

piggy backing onto an existing breaker, definately a no.
If you connect directly to the meter tails it should be via a suitable device such as a Henley block and then onto a sub board with your overcurrent protection.

 

[paulsamuel1984]

Thanks Moggy 1968,

The following are the method of connections I am conecerned about and my thought process. Any thoughts appreciated.
1. 50kW Connection directly on to mains isolator terminals (MCB & RCD protection provided). - I consider this acceptable, large isolator so Henley Block not practical.
2. 50kW Connection piggy-backed onto existing breaker - I consider this non compliant.
3. 50kW Connection directly on to mains isolator terminals (ONLY RCD protection provided). - I consider this acceptable, large isolator so Henley Block not practical. BUT MCB is required.
4. 35kW Connection conected directly onto sub-main cable feeding a Building Services DB (RCD Protection only) - Not sure about this one!!

Any thoughts would be appreciated before i get my sub contractor to change.

PS

 

[moggy1968]

I'm stepping a bit outside my sphere of expertise here, I'ma G83 guy! I am sure others will have a more informed view.
in my view
1. ok but the RCD musn't be shared with any other circuits. There have been lengthy debates on here elswhere about wether a type B RCD must be used.
2. non compliant
3. RCD as above. also the RCD offers no overload protection so you need an MCB or type B RCBO (type B on the RCD element as well as the MCB side. most generally available RCBOs don't have this)
4. As above

 

[paulsamuel1984]

Hi Moggy,

Thanks for you help. I think we are on the same page with our thoughts on that. Looks like my sub contractor might have some tweaking to do!!

PS

 

[Earthstore]

Also not quite my thing, but I see no mention of G59 protection.
Did the DNO not specify separate G59?
I am aware that some inverters have G59 protection built in however some DNO's insist on separate G59 protection.

I hope this helps

 

[moggy1968]

appologies for my ignorance, as I say, I'm a G83 guy, but what is G59 protection?

 

[paulsamuel1984]

No G59 protection required as the inverters are g59 approved and installs are under 50kW. All confirmed with DNO. It is undervoltage/overvoltage protection. Different parameters compared to G83.

I am engrossed in this topic now and I have picked something else up. Reg 551.7.2 implies that you can install on circuit that is not dedicated as long as you meet certain requirements (I will not be doing this anyway). However, it is my assumption this is for FINAL CIRCUITS only, for example, if I was to use a splitter box on a sub main cable (protecting PV with its own MCB & RCD) I assume this to be OK although it is on the load side of the breaker upstream of the sub main.

Alot of assumptions there!!

 

[Earthstore]

Moggy, as I understand, G59 protection tends to be more for 3 phase, if there is a fault on one phase, over or under voltage, the whole system will shut down, this happens for both import and export, it protects both PV system and the grid if it is a reverse fault. Also it would protect 3 phase equipment.
Also if you have to disconnect a system that has manual G59 protection you must shut down all three phases at the same time (not sure if that is common practice anyway) or you will trip the G59
Sorry I am not a sparky, so do not shot me down on this one chaps.

I hope this helps

 

[BruceB]

G59 in practice is usually 3 phase, but applies equally to single phase if over 16A.

On 551.7.2, it is a general case for generator sets.
But I would contend that the specific pv regulation 712.411.3.2.1.1 stops you putting it on a shared final circuit.
Even if you could, you would be unlikely to meet 551.7.2(v) which requires the generator to stop producing >50V within a certain time.
DTI guide at 2.3.1 also says separate circuit.

However, no particular problem sharing a distribution circuit (ie sub-main). The sub-main would need to have appropriate fault protection of course.

 

[paulsamuel1984]

BruceB. If sharing a sub main, am I right in assuming the pv circuit will require its own fault protection and cannot rely on the protection of existing circuit breakers on the sub main.

 

[BruceB]

The branch in the sub-main to the PV distribution board is part of the sub-main so protected by the sub-main circuit breaker. You would need to ensure the branch in the sub-main to the PV distribution board was a suitable size for the existing breaker to provide fault protection. If too small or too long then it would need its own mcb.

 

[paulsamuel1984]

BruceB,

What would you consider to be too long a length fro the branch to warrant its own breaker? The cable size agaisnt the existing breaker is acceptable.

And last one for everyone: I know you cannot piggy-back onto a final circuit breaker, but what about a sub-main breaker (i know it not good practice). Presumably the DTI guide still applies and it cannot be done.

PS

 

[BruceB]

By too long, I mean that the Zs at the far end is too high to cause the mcb to trip within the TN time or TT time for a distribution circuit. In which case you would need to add an rcd. But if you are anywhere near those limits you have probably gone way over the 1% voltage drop limit which will cause other problems.

I see no problem piggybacking onto a sub-main breaker provided:
- the sub-main breaker (and rcd if there is one) provide adequate fault protection for the piggybacked bit; and
- there is a separate breaker downstream to define/protect the pv final circuit.

It is not standard and without double checking, the DTI guide might not like it, but it would not be against 7671 in my opinion.

Regards
Bruce