Nick
I think you mean arc resistant gear, not "arc rated".

If there are no sensors installed, it's possible someone else will discover it "the hard way".

A common misconception about the arc-resistant switchgear is the purpose of its design.
Arc-resistant switchgear is designed to contain the arc inside the switchgear and exhaust the gases that result from the arc through a plenum to keep the workers around the switchgear safe. However, all of this is meant to happen when the switchgear is closed. When the switchgear is open, it loses its benefit as arc-resistant! This means when someone is performing maintenance, thermal scanning, or even if the switchgear panel is missing a bolt, danger is still a factor.
Although having arc-resistant switchgear increases safety in some instances, be prepared for incidents that happen when someone is working with live equipment. It’s best to identify the hot spots by performing a study, reduce the incident energy levels at those spots using arc flash mitigation solutions and/or proper coordination, and keep maintaining the equipment periodically in order not to miss that bolt somewhere!

Converting energy to motion for more than half a century

Dear Mr. NickParker (Electrical)(OP)10 May 22 15:53
"... #1. On an arc rated MV or LV switchgear, are we safe from the arc flash point of view when we open the LV compartment door? #2. If the switchgear has been tested for arc rated, the arc flash detection sensors need not be provided. Is my understanding right?...".
#1. No. Not save when any of the LV or MV/HV compartment door/side-plate/back- plate/top-plate etc., is open. Note: arc resistant gear are tested with all doors closed with proper locking device, side/ back / top-plates are properly bolted down. See IEC the basic five criterial.
#2. In all cases, irrespective of whether LV/MV/HV arc resistant or not, it is advisable to install the arc flash detection sensors at a veery small cost in comparison to the cost of the board/safety of operation personnel.
Note: 1. The arc flash detection sensors is to trip the up stream/incoming breaker instantaneously (without time delay) when an arc is detected.
2. Attention: With doors open, side/back/top plates removed, the [older model] arc flash detection sensors [without over-current sensor incorporated] does send a tripping signal with flash created by photographic flash. However, the new models over come the problem.
Che Kuan Yau (Singapore)

Medium voltage arc resistant gear might be built so the low voltage compartment door can be open while still maintaining the arc resistant rating.

Regarding photographic flash - I've never yet been able to trip a flash sensor using a built in camera flash. I have to use a dedicated flash unit to get a bright enough flash.

Dear Mr. LionelHutz (Electrical)13 May 22 11:54
" #1.... Medium voltage arc resistant gear might be built so the low voltage compartment door can be open while still maintaining the arc resistant rating. #2. Regarding photographic flash - I've never yet been able to trip a flash sensor using a built in camera flash. I have to use a dedicated flash unit to get a bright enough flash."
#1. Agreed, if the LV compartment is fully [pressure] sealed from the MV compartment. Attention: It is difficult to ensure that the pressures does not throw out the LV devices in it, during the test.
#2. I had personally demonstrated it in Singapore that an [old without overcurrent incorporated] arc monitor did send a trip signal with a remote photographic flash at a distance of about 3m away. I would think that it depends on the sensitivity of the monitor, the strength of the flash, the distance and the angle of the flashing light. As mentioned, the new design with [light+ over-current] overcome the problem.
Che Kuan Yau (Singapore)

For the LV compartment on MV switchgear, it is possibly the case that you can open the LV compartment without compromising Arc Fault Containment.

Best to contact the manufacturer. However, a general intuition may be given by the form of enclosure fastening.

If the MV compartment has a series of torqued bolts, but the LV compartment has only a rotary tongue lock, it is probably not part of the Arc Fault Containment.

Probably an old relay that was too sensitive. I looped a bare fiber about 15x into a SEL relay and flashed it with a digital camera flash multiple times with the flash right against the fiber and could not get a trip. The dedicated flash causes a trip with a single pass of fiber.

You do know that most arc-flash relays must also see an over current at the same time as the flash?

Not when I program it to trip on light only.