110v MCBs/RCBOs

Tuco88

I have crossed this bridge before on a site years ago, and I cant think back to the solution that was given :rolleyes:.

1)So its not an Isolation traffo, I thought it would solve my issue by doing away with it. Its a C.T.E, we have installed new 20Amp RCBOs a like for like job. But now the test button will not operate unlike the old ones(Note 230v RCBO) I can only think that it has to do with the test circuit not having enough to operate it at 110v as its mA operated does the voltage really matter? Which takes me to a testing problem that our fluke 1653 is unable to test 110v sockets. Anybody know of a make that can?

2)Is it OK to use 230v MCBs on 110v applications? I can't seem to find them too handy, IMO it should be fine as these devices are current dependent more so than voltage?

Thanks for you thoughts.

@110V

I=V/R
afaik the test throws a series resistor between phase & neutral so perhaps the resistance it too large for 30mA @110V.

air

On 1) I think Liam is probably correct with respect to the test circuit resistance not being low enough for 110v testing operation.
The voltage between live and earth is only 55V as opposed to the normal 230V so the test fault current is going to be 4x less than designed.
You'll need to find a 110V specific tester.

On 2) The MCBs will be fine but you'll need bigger conductors so that your fault loop impedance is low enough to activate the MCB within the required clearance time. I believe you are still required to clear within 0.4s.
110V circuits are going to be cabled in twice the normal CSA anyway for equivalent appliances.
So just test the circuit fault loop impedance as normal and check the trip curve for the MCB to ensure it will clear in less than 0.4s at 55V.

Steve

Why are you using RCD's on a 110V CTE system?

My understanding on the concept is that CTE 110V is considered non lethal so they are not needed. Has that changed?

As far as tripping is concerned, it is purely current based, although as air said you need to look at the trip curve vs the traffo impedance. Breaking capacity will be minmal unless it's a huge traffo.

Bruthal

Sir Liamalot wrote: »

I=V/R
afaik the test throws a series resistor between phase & neutral so perhaps the resistance it too large for 30mA @110V.

That's the only real difference in an operational sense. The test button connects the resistor, bypassing one set of current coils. Even a 1.5v battery connected across L to L or N to N can sometimes trip the RCD just as the battery is connected. So for actual RCD operation, it's all the same.

air

Yep, Steve is spot on.
Ideally you need to measure the supply impedance at the input (230V) to the transformer,calculate the reflected impedance at the output and add this to impedance of the 110V windings and the outgoing circuits. The total figure is then the fault loop impedance.
In practice this is likely to lead to impractical cable sizes for the final circuits by the letter of the law.

You will face the same issues with circuit protection on 230V UPS or off grid inverter circuits, where the power supply source can't supply enough fault current to clear protection devices within the specified time.

air

air wrote: »

You will face the same issues with circuit protection on 230V UPS or off grid inverter circuits, where the power supply source can't supply enough fault current to clear protection devices within the specified time.

What's the usual solution to this Air?

air

Hi Liam,
I don't know to be honest and it's something I've been researching fruitlessly for a while.
The only answer I'm seeing is small MCB sizes or RCD protection on all outlets.

SMA advise a maximum outgoing circuit breaker size of B6 on their 3kW off grid inverters and a maximum of B16 or C6 on their 8kW models. Obviously this is impractical if you wanted to use them to supply large inductive loads for example.
Ideally one would use breakers with a trip curve specifically tailored for the high source impedance but I don't know if they're available or even practicable.
I think the approach adopted tends to be to use RCD protection on all outgoing circuits for shock protection and just rely on normaly sized breaker ratings for longer term thermal tripping.
In reality I don't think many people are aware of the issues these supplies cause.

I've noticed...bittova nuisance. So people can only get 1.5kW continuous from their 3kVA SMA?
I'm fighting with a 2.5kVA atm.

air

As I said, I think in practice people use RCBOs and just work away.
The bimetallic thermal trip will protect against long term overload as usual and the RCD portion will protect against shock.
The magnetic trip won't operate but it's not such a big deal so long as the RCD works!

2011

air wrote: »

The magnetic trip won't operate but it's not such a big deal so long as the RCD works!

Why? Because the transformer limits the magnitude of the current enough to prevent the MCB magnetically tripping?

You can't get the over-current from an inverter with a short circuit to operate the instantaneous trip. It just doesn't have the grunt for 5X rated

2011

Sir Liamalot wrote: »

You can't get the over-current from an inverter with a short circuit to operate the instantaneous trip. It just doesn't have the grunt for 5X rated

Apologies, I thought you were discussing a transformer (following on from the opening post).

Anyway, the same applies: If an inverter or transformer were to limit a fault current (due to a "lack of grunt") resulting in the magnetic component of an MCB not operating, does it really matter "in the real world" ?

Well it's arguable. Certainly being advised to put smaller breakers than the rated load on a device is annoying.
Most of these things come with internal over-current protection anyways.

I'm actually fighting with a proprietary serial data interface but wasn't interested in hijacking the thread.

air

Apologies for the confusion caused.

The magnetic trip is there to protect against short circuit faults, with it effectively disabled you have to consider what might happen between the short circuit occuring and the thermal trip activation. The thermal overload trip is the only protection left unless there is a simultaneous earth fault which triggers the RCD.
In theory you would calculate the prospective short circuit current allowing for the source & final circuit impedance, then check how long the thermal trip will take to trigger at that current.
Finally work out the temperature rise in the cables & see if that presents a problem for the cable insulation.

I'd give it a go but I'd be concerned about frying the output MOSFETs on my off grid inverter
I think it would be a bigger issue on lightweight high frequency inverters as they would have the highest source impedance.
Unfortunately modern UPS units fall into this category.

Going back to the original question, this is a bigger issue with small transformers that will present high source impedance.

air

2011 wrote: »

Why? Because the transformer limits the magnitude of the current enough to prevent the MCB magnetically tripping?

Yes, it will likely be the case for a small transformer also.

2011

air wrote: »

Yes, it will likely be the case for a small transformer also.

The voltage will drop, the current will rise and the bimetal will cause the MCB to operate due to sustained overload after a period of time.

I have seen special quick blow fuses, such as semi-conductor type used on large UPS units in the past.

air

air wrote: »

I'd give it a go but I'd be concerned about frying the output MOSFETs on my off grid inverter
I think it would be a bigger issue on lightweight high frequency inverters as they would have the highest source impedance.

That's exactly the problem.
Mine's just back from service although I managed to do it with no load.

High frequency inverters are pants. I may flog this one...I was duped by the schlick sellsheet.

2011 wrote: »

I have seen special quick blow fuses, such as semi-conductor type used on large UPS units in the past.

Aka MOSFETs :pac:

2011

Fuses like this:

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_High_Speed_Fuses.pdf

air

2011 wrote: »

The voltage will drop, the current will rise and the bimetal will cause the MCB to operate due to sustained overload after a period of time.

I have seen special quick blow fuses, such as semi-conductor type used on large UPS units in the past.

Yes, obviously it will blow, it's the time taken that's potentially the issue.
Quick blow fuses ideal for the likes of a UPS, especially one with an old style bypass that negates the issue apart from during the brief occasion when running on battery.
The odds of a fault occuring while the UPS is in circuit would be tiny.

Quick blow non resettable fusing could get tiresome in a site environment however.

2011

air wrote: »

Yes, obviously it will blow, it's the time taken that's potentially the issue.

This is down to fuse selection.

Quick blow non resettable fusing could get tiresome in a site environment however.

Why? Short circuits should be a rare occurrence.

2011

kind of of topic, but I recently selected Phoenix Contact quint power supply units for a project as they have a "dynamic power reserve".
This allows the PSU to supply up to 6 times the nominal current for 12 ms to assist to the fast operation of fuses / MCBs under fault conditions.

air

2011 wrote: »

This is down to fuse selection.



Why? Short circuits should be a rare occurrence.

I agree it's down to fuse selection but the discussion is becoming less valuable now as the relative issues are going to be dependent on the particular hardware in use and the results of situation specific calculations.

True short circuits may be relatively rare but things like a stalled motor start (saw blade jammed by a chip of wood for example) are not rare and almost as likely to take out a quick blow fuse as a dead short.