APC Smart-UPS and BS7671/BS7909 disconnection times

[dimitrisb]

hi

What would be the Automatic Disconnection Time for a short circuit fault on the load side of an APC UPS?

In the scenario that there is a L/N short on the load side of the UPS, the breaker feeding the UPS will trip but then the UPS battery power will kick in right? So the UPS will see the short as an overload and turn itself off.

Would that meet the requirements of SC disconnection times of 0.4s?

Thanks

[Teken]

That depends upon many factors such as the type of failure mode and the amount of current flow. 

The two most common failure modes are a slow & random partial short. The other is a catastrophic and immediate dead short.

This than comes down to how sensitive the UPS output circuit is and how fast the relays are designed to react.

Only a validation test can determine how a specific unit will behave. Keep in mind presenting a dead short on the output can and may cause the unit to fail! ☝️

As such, using a regulated power supply to limit the maximum amount of current draw is imperative. 

Questions Ask . . . 👍

[dimitrisb]

Thanks for the reply. 

The reason I'm asking is because I work for a hire company for events and we need to comply with the regulations set by BS7909 which require Automatic Disconnection Times on final circuits of <400ms for L-N Short and <300ms for Earth Fault.

Earth Fault is not as much of an issue as with an RCD we only need about 30mA of Prospective Fault Current and the UPS being a floating supply when on battery mode.

The issue is with L-N short. We will have to measure the Prospective Short-Circuit Current at the final circuit and make sure that the protective device will disconnect at <400ms.

Say we get 300A of PSC measured at the female end of the appliance lead feeding a unit connected to the UPS.

If there is a L-N short on that unit/appliance lead and the UPS is on utility then no problem, the C16 MCB/RCBO feeding the UPS will disconnect in <400ms.

The issue is that then, the UPS will kick in due to loss of supply. Will the battery mode UPS, in turn and presented with a short and 300A, disconnect within 400ms to meet BS7909 requirements?

We could get around the issue by placing an MCB inline on the UPS outputs. Then we will need to know what the MAX current flow of the inverter would be so we can select an appropriate curve/rating MCB to achieve ADT of <400ms. If for example we know that max overcurrent of the UPS is 20A we would have to install a B4 MCB on the outs to meet BS7909 requirements.

So I'm guessing the manufacturer should be able to provide this information? Or as you said we will have to experiment!

Specifically I am referring to our hire stock UPSs

APC SMC1500I-2UC

APC SMT750RMI2U

And the older

SUA1500RMI2U

Thanks for any advice!

[Teken]

You may review this APC FAQ as it relates to Short Circuit on specific models and the stated behaviour:

https://www.apc.com/us/en/faqs/FAQ000247935/

I suggest you reach out to APC Technical Support for any relevant technical information pertaining to short circuit protection.

More specifically the *White Paper* that discusses how APC addresses this issue. As the support guy will probably have no idea besides referencing the external breaker / internal fuse rating. 

Keep in mind a UPS by its very design uses a slow b-low fuse / slow reaction breaker. 

There are no GFCI rated (output) UPS’s in the Enterprise market due to its inherent use case and intended purpose. 

Lastly, if there is a short detected on the output the system will call this out. How the system reacts is determined by the design topology and the type of short presented to the system.

You’ll find that some units do nothing besides alert you of a output short. Whereas others will terminate the output voltage upon detecting a short.

Again, this varies from model to model and firmware installed on the system.

Lastly, do not assume using a resistive load is the same vs a reactive one when validating a system under test. ☝️

Regardless of all the above a solid low resistance single point Earth ground is essential for equipment and human safety. 

Questions Ask . . . 👍

[dimitrisb]

Thank you so much for all the advice!

IF it's slowb-low type fuse/breaker on the UPS then I'm guessing an MCB on the outlet of the UPS would react before the UPS had any chance to react if presented with a L-N short as an MCB would trip in less than 100ms all the way down to 5-10ms.

Will do further enquiries.