Date: 201704
Local time of day: 1201-1800
Location: ZZZZ.Airport, FO
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Cabin Lighting
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Passenger
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: My 'A' Flight Attendant (FA) chimed the cockpit informing us of an electrical smell mostly emanating from the forward most part of the passenger cabin; on the passenger side of the forward galley. As both pilots were listening up on the service interphone; my First Officer (FO) pulled out his O2 mask (as I did); and used it while I further questioned the FA. At no time were there any noticeable vapors or fumes circulating on the flight deck; therefore smoke goggles were not needed. While I asked the FA to confirm it was an electrical smell; my FO sent an ACARS message to Dispatch initially of the fumes reported. I asked if she knew its source; which she did not. I then asked her to question the passengers in the area the fumes appeared the strongest about the contents of their baggage. This included having her check the overhead bins in that area as a possible source; with emphasis on laptops; cell phones; other electronics or power sources; while we initiated our protocols for a report of fumes being reported in the cabin. I asked her to notify me of her findings; and if the fumes intensified or decreased by our actions from the QRH. We then pulled out the QRC and accomplished the items under Smoke; Fire or Fumes. She called back shortly after this and said the fumes were intensifying. We continuously evaluated what was happening; and in an effort to better identify; process; and comprehend her information; I said 'I have the aircraft'; and sent my FO back to the passenger cabin to make an assessment (this involved coordinating the safety zone). My FO had the passengers in the affected area help him assess for possible sources of the fumes. Consensus was drawn from the Cabin Crew and those passengers in the affected area; the fumes they were experiencing were of an acrid nature. My FO returned to the flight deck to report his observations; and then the FA chimed again; except this time she and the 'B' and 'C' FAs thought the acrid fumes were even intensifying more. I decided to inspect the area of concern for myself (using safety zone protocols); and the fumes were very noticeable to me around rows 4; 5 and 6. The passengers in these three rows were very alarmed; and it was evident to me the existing fumes were creating a physiological reaction to those affected. My concern was without eliminating the source of fumes; there would be a medical emergency and a fire as a result of persistent fumes if the flight continued. It was evident that a continued flight was not the safest course of action. As the QRC leads us right into the QRH; we executed the checklist items (turning off non-essential electrical power; etc.) in hopes to eliminate the source of the fumes; except these vaporous emissions of fumes (while colorless to the naked eye); were not decreasing. My 'A' FA chimed again saying it was their belief the fumes were getting even worse than earlier reported. My short exposure to these fumes made me believe a longer exposure to the passengers would be harmful. Its source was unknown. The QRH states 'Fumes that occur in the cabin constitutes a fire onboard the aircraft.' We evaluated this situation as an emergency. Having a most suitable airport in our proximity; in point of time and distance; we chose the safest course of action; given the circumstances; was to divert to [a nearby airport]. I radioed Center with our 'PAN PAN PAN;' and asked for their help clearing us direct.Explanation was given to ATC for our reason to divert. Clearance was given; and the Pilot Flying updated the FMC to [the new destination]; and executed it. A PA was made informing the passengers of our decision to divert; and the reasons for doing so. We did not delay landing in our attempt to complete the Fumes Removal Checklist; and all things considered; we made every attempt to accomplish all checklist items in a deliberate Challenge-Response-Response method consistent with managing intensive tasks.Continuing to Maintain Situational Awareness; we enlisted our Dispatcher to confirm our landing performance; but we were able to accomplish the Descent and Approach Checklists ourselves; as appropriate; with the current field conditions (ATIS). Upon getting a hand off to Approach; we were asked for souls onboard and fuel remaining. We requested a clearance direct to ZZZZZ Intersection for the RNAV [approach]; this allowed us the needed time to brief the arrival (landing was below MLGW). We asked the Tower Controller the frequency the Airport Rescue and Fire Fighting (ARFF) was on; and we were told on the Tower frequency. We confirmed ARFF was on standby. We stopped the aircraft on the runway for a safe landing and set the parking brake (I made a PA asking the passengers to remain seated); and ARFF communicated there was no smoke or fire. The Tower then cleared us in to our parking bay.After engine shut-down; ARFF pulled up air stairs to the FWD passenger door where I met the Battalion Chief; and at the same time ARFF inspected the cargo bins. He asked for all passengers to deplane; but as it appeared the fumes had dissipated; and there was no sign of smoke or fire; and due to the immediate ramp being congested with other local aircraft; including the danger of moving propellers and gusty winds; I asked the Fire Chief if his safety inspection of the passenger Cabin could be accomplished with all passengers remaining in their seats; and he agreed. I made a PA from the FA station informing the passengers to remain seated while ARFF conducted their safety checks. After it was determined the passengers could be properly accommodated in the small terminal; and with the help of the Station Manager (for the new station not yet opened); an orderly deplaning of the passengers commenced. The aircraft was met by a (other carrier) B-737 Mechanic (with the assistance of my Dispatcher and the Chief Pilot in the Operations Center); and the aircraft was secured for the night.Reportedly a bad ballast in a cabin light (logbook corrective action to make it airworthy). If it even was a faulty or failed ballast if there was a current spike overheating a ballast of a light without a circuit breaker trip; where is the fault detection capability; or fail safe to prevent emissions of harmful fumes?


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737NG flight crew reported diverting to a nearby alternate after electrical fumes were reported in the cabin.